Because Sirius Black needs some love.
They knew each other when they were in diapers. There were distant cousins and their families were good friends. They don't remember meeting, just that they were always There.
She makes fun of his French and he makes fun of her calligraphy. Her father is French so while it comes naturally to her, he struggles. She teases him in the language he struggles with, giggling when he turns bright red in frustration. This teasing continues on into their school years, even after he masters French but gives up calligraphy. She sends him owls during school with taunts written in elaborate French and he responds in messy French. Their friends throw each other knowing looks when an owl arrives but they ignore them because it will never be like that.
Her first bit of magic comes when she's five and sick. When he finds out about the spotted bubbles she coughed up, he bursts out laughing; she sends him flying into a nearby tree. He seeks revenge a few months later when he figures out he can turn her hair pink if he just concentrates.
At first she's jealous that he has a little brother, someone to play with on a rainy day. She passes the days when she doesn't see him in her father's library; she becomes very fond of the old dusty books on the shelves. He passes the days by playing little pranks on his brother; he later hones this talent in school.
They both realize something is wrong with their families at age six when they start studying their family trees. While the tapestry has been hanging in his house for ages, he never actually looked at it. He tells her about the family members who have been blasted off. They question their parents and listen, horrified, as they start spouting off nasty words and comments. They wonder what's so bad about the blasted off cousins and uncles—they're family after all. Is it really so bad to do something different or be different?
They start paying attention then and notice that the family is always saying mean things. They grow up hearing about the 'evils' of Muggle-borns. But they hear stories from their cousin, whispered stories in a room far away from the adults; they learn they're not so bad. They listen, captivated, by stories about Hogwarts and its professors. Their cousin tells them about the different houses and one stands out in their minds. They go to bed that night, dreaming of lions and scarlet and gold.
His letter arrives first. She's upset when he comes over and shoves it into her face and runs to her mother, begging for her letter. When it finally arrives, they all go shopping together and stand, feeling important, while being fitted for their robes. When she finds her wand, she waves it at him proudly, accidently turning his ears bright red. He gets a midnight black owl and she, a white cat that curls up at her feet in bed. They both can't sleep the night before September 1st. He sneaks down stairs, careful not to bother the sleeping portraits and determined not to look at the heads hanging on the walls. He goes into the tapestry room, runs his hand over the green and silver thread, knowing it's not what he wants. She sneaks into her father's library and reads her favorite fairy tale to calm her verves.
He abandons her on the train to sit with a boy who has as much energy as him. She sits in an empty compartment and doesn't look up when a boy with greasy hair and a girl with bright red hair walks in. They're complaining about something but she tunes them out, shifting in her seat to get more comfortable and keeps on reading.
They're both sorted into Gryffindor. He's in shock, unable to believe it; he ignores the looks from his cousins. He receives a scathing howler from his mother. Her mother is also angry but her father is French and their customs are different. Still, her father listens to her mother; it hurts her to have disappointed him but she can't keep the smile off her face when she draws the scarlet curtains of her bed every night. Every morning they descend the stone stairs into the scarlet and gold common room and hear the laughter of their new friends; this is where they belong, they realize.
The girl with the red hair comes to be her best friend. Her parents find out she's Muggle-born but her father allows the friendship—for now; he's sure she'll grow out of her childishness one day. His parents are content that his best friend is at least a Pureblood. For now everything is alright.
They pass their first few years without any incident. They both have different circles of friends but every once in a while, they'll share a glance or a nod of understanding. When he receives angry letters from home, he seeks her out in comfort. When her grandparents die, it's he who she goes to first. Sometimes, when he and his friends have a particularly hard time planning a prank, she'll lend a helpful hand, always discreet lest her best friend find out.
They start to grow apart during second year. At first she doesn't think anything of it; they have different friends and different interests. But as the years go by and he and his friends get more secretive and he stops going to her for help less and less, she starts to worry.
The summer before fifth year, their parents make an offhanded remark of what a lovely couple they would make. The thought horrifies them both. Though they haven't actually said anything to each other, they hate their Pureblood status and would be more than happy to cast it off. How could they do that if they're together? She hates that her parents want to arrange someone for her and he hates that his parents are still pretending to care about him.
Despite this, they can't help but notice how they're growing. He is very handsome, she realizes one day in the corridor. Sometimes, when they pass each other, a glance lasts too long and she's unable to look away from his gray eyes. Sometimes in the common room, he'll look up and see her sitting by the fire. He loves how the light bounces off her dark curls. They hide their attraction from their friends and each other, unable to face it or truly admit it.
At the end of fifth year she's torn between him and her best friend. In the end she chooses her best friend and ignores him for the rest of term and all through the train ride. Sometimes she wishes he would just grow up and stop pulling pranks. The rest of the time she wishes she could not care like him. He just misses the way she steals glances of him.
That summer he runs away and is disowned. She's locked up at home, her parents worried she'll do the same. He writes to her but his letters are stolen, sometimes before she can even read them. She's forbidden from writing back but she does it anyway, sending the letters to her other friends to pass along, forgiving him in them.
They're glad for sixth year, when they can pretend they're normal again. Though he's just as distant with her as before and disappears with his friends more than ever, she's glad she no longer has to hide her friendship from her parents. He's happy he can watch her by the fire again. At the end of sixth year, he has a falling out with his friends. He messes up, big time. But she's there for him; her respect for him soars as he reveals his deepest secret. Secretly, her heart swells for his loyalty to his friends. She goes to his friends and coaxes them all to make up. Eventually it works and she gives him up to them again.
Another family member is blasted off the tree for giving him inheritance money. He buys a flat and invites all of his friends over, including her. She's impressed by the size and the fact that he actually bothers to furnish it. When he gives her a grand tour, her eyes linger on the bed too long; her mind wondering what it would be like to stay there. He sees this but swallows and doesn't say anything.
Seventh year is their best year. Their best friends finally start dating and they couldn't be happier. Their two groups turn into one and several times during the year they can all be seen laughing together at a table in the Three Broomsticks. Sometimes though, he'll sneak her out for a drink between just the two of them. Their friends pester them, wanting to know when something will Happen. She brushes it off, saying she doesn't want a Pureblood. They're just friends. But she's lying to herself. He shrugs, not really saying anything on the subject.
It happens two weeks before they graduate. She sneaks up to the astronomy tower, seeking solitary in her last few nights in the castle. She feels the weight of the world—of the war—crashing down on her shoulders. Though her parents favor Purebloods, they are determined to stay outwardly neutral. Her father wants to protect the family name and fortune by going into hiding in France. She doesn't want to go; she wants to protect her friends and her beliefs. He finds her there, staring up at the stars. He follows her gaze and sees Orion—and the star he was named after. It wasn't the map that let him find her—that was confiscated months ago—it was his own desire to see her. He knows about her family and he doesn't want her to go. He takes a step forward and, noticing the movement, she turns and sees him.
At age seventeen, she's the most graceful woman he's ever seen. Beautiful. Her long ringlets whip around in the warm breeze, strands covering her face. He takes another step forward and reaches out, moving her hair aside, but doesn't take his hand away. She brings her hand up and covers his with hers. Gray meets blue as years of untold love and desire passes between them. Finally he closes the gap between them and kisses her.
They move in together after graduation. Her parents threaten her with disownment but she stands her ground, saying that at least she's with a pureblood. Eventually her parents give up and leave for France. They both join the Order and fight for everything they believe in. Sometimes she cries herself to sleep in his arms, pained by the death she sees around her.
They attend their best friends' wedding—he the best man and she the maid of honor. As their friends exchange vows, their eyes meet. Hers are filled with tears, moved by the love between her two friends and her love for him. He smiles, feeling the same. At the reception, they dance together and share kisses between them. Her best friend has a baby and they're godparents. They couldn't be more honored as they go to St. Mungos and visit the new parents. She gets to hold the baby and as she cuddles it to her, she looks up, a delighted smile on her face, and meets his eyes. He looks strange but happy, more than he did at the wedding. She doesn't know that he wants to Ask her soon.
He doesn't get a chance to, though. The war continues around them and she's crying more than before. One of her dear friends is killed and she's almost inconsolable. The months drag on and he promises himself he'll do it soon. Finally, during Christmas, he does and she doesn't hesitate to say yes. The war takes up their time though and secretly they both think they're running out of time with each other. He wants to elope but she insists on doing it properly, with the friends they have left. He doesn't have to heart to say no to her.
When it's the baby's first birthday, they all celebrate. It's the last time they ever have so much fun.
Their friends go into hiding. She thinks he's the Secret Keeper—he doesn't tell her he switched, wanting to protect everyone. She's scared out of her mind. One night he leaves and doesn't come back. That's when she Knows.
She doesn't know what to do; she can't believe he would betray his friends—her friends. She knows he wouldn't. She's questioned by the authorities and by her former headmaster. She can't tell them anything because she doesn't know anything. He gets carted off to Azkaban. She is believed to have been a possible conspirator but they can't prove anything. This doesn't stop them from forbidding her from seeing her godson and exile her to France, to where her parents have come out of hiding. She's locked up in her parents' home, a prisoner in her own right.
The years pass slowly. She wonders about him and about their godson. She mourns for her friends, every one of them. She mourns him, how he's wasting in his cell. He can't think about her, she's his only happy memory and he's determined not to have it taken away.
She hears about his escape from Azkaban and her heart is overjoyed. She breaks down into tears in front of her parents but she doesn't care. He thinks about her, truly thinks about her, for the first time in twelve years. He breaks down into tears when her beautiful face comes to mind once again.
The minister comes to visit her but of course she doesn't know anything. She doesn't tell him his secret; she wouldn't betray him like that.
After a year of being free he finds out where she is and visits her, deep in the night. She almost doesn't recognize him but she knows it's him. She embraces his thin form, silently crying over what has happened to him. He holds her, barely able to believe it, and tells her everything. He can't stay long though; he knows she's being watched and leaves that same night. She never sees him again.
He's killed two years later, murdered by the cousin he despises the most. Even her parents show their sympathy for her when they see her clutching her engagement ring. When he's finally proven innocent, she is, too. Though it was a place of so much pain for her, she returns to England and once again joins the battle.
She has few friends left after so many years. She doubts they'll remember her but they do and welcomes her back with open arms. She meets her godson and, like so many others, marvels at how much like his father he looks. But with his mother's eyes. She's sorry, sorrier than she can ever say, that she hasn't been there for him.
Eventually, after two years of hard fighting, the war is finally won. More friends are lost and she's the last one standing from her group, but she stands proudly, cherishing their memories. She watches as her godson and his friends fall in love and get married. She envies them, heartbroken that that was taken away from her. But she would never take it away from them, never. They deserve it.
She goes through her life quietly, working as Healer at St. Mungos until she is offered the position at Hogwarts. She accepts, bound to the place that holds her best memories. Sometimes she can hear his laugh through the halls and sometimes, when she looks at Gryffindor table, she sees them, her friends all together once again.
When she finally passes years later, he's waiting for her. He looks young, like the carefree boy he was during their school days. Her curls are dark and tight again, forever young. Her friends are all around her, surrounding her and embracing her. And finally, finally, she is his.