Obsession
Sirius had been in love with James for as long as he could remember knowing him. Sometimes Sirius thought he had loved James ever since their first train ride together, but that was idiotic. Sirius didn't believe in love at first sight. Sirius had certainly liked James a lot since their first train ride together though, and it was that train ride that had formed their friendship, the whole structure of their lives at Hogwarts. Their friendship with Remus and Peter (damn the rat), and their rivalry with Snape.
They had started out as friends, formed their own little group, the Marauders and they ruled the school. They had all been equals; Remus and Peter were never just the followers even if they had followed more than they lead. There was never any doubt that James and Sirius were the leaders, the coolest and the best.
It had started with James' first crush a girl named Lucy. Ravenclaw. Even Sirius had to admit that she was quite pretty and funny in a strange sort of way, her fondness for strange experiments aside. There was just one problem. Something that he tried desperately to ignore but that kept on bugging him when he couldn't think of anything to distract him. Sirius was jealous, burning, green flamed jealousy. James, in Sirius' head at least, was his.
Sirius had known he was gay for a long time. For a while he had hoped he might be at least bisexual so that he could at least fulfil some of his family's wishes and marry well, but try as he might, he could just not get attracted to girls. He could admire them well enough, like an art lover admiring a painting, but he could not desire them like other boys did. It seemed to Sirius that he was built purely to anger his family. Sometimes that thought pleased him, other times it scared him.
Then came Lily. Lovely Lily. Perfect Lily. The really annoying thing about Lily was that Sirius actually liked Lily. He didn't think that the sun shone out of her every orifice like James did, or that her hair was like cascading fire (James' poetry did get better with practice. Lots of practice), but Sirius liked her. Lily was funny and smart, not quite the goody goody two shoes that so many thought she was, in fact she could be quite the mischief maker when she wanted to be, and she was the most loyal of friends. She didn't even hesitate in accepting Remus when she found out about his 'furry little problem' as James termed it, which was a big plus point in Sirius' book. Although, when Sirius thought about it, and he tended to obsess over it, her acceptance might have been more to do with the fact she was Muggleborn and not brought up with the same fears and prejudices as those from wizarding families, than her generally kind nature. Whatever the reason, Remus' face when she told him she still liked him whether or not he turned into a bloodthirsty beast once a month was enough reason to like her.
It took Sirius a long time to find the balance. He had realised a long time ago that James would never love him like he loved James; James just wasn't built that way, just like he wasn't built to like girls. Sirius had accepted that and as long as he was still the most important person in James' life he was happy. For a long time he was. They were best friends and he was James' greatest partner in crime. The joint Kings of Hogwarts. Lily upset all that. Not deliberately of course, Sirius couldn't even hate Lily for taking James away from him because for so long Lily Evans had wanted nothing more than to be rid of the tiresome attentions of James Potter.
It was hard, however, for Sirius to be reasonable when she got his soft looks, his longing sighs. His kisses. Sirius found his balance though and he ended up loving them both, meaning, he didn't love Lily like he loved James. He couldn't, obviously, but she was like a sister to him. They were family, and he loved her like family. Anybody who hurt Lily would face the wrath of both James and Sirius, never mind Lily's ever so slightly annoyed insistence that she could look after herself. Whenever Lily would bring it up Sirius would insist it had nothing to do with sexism – if anybody hurt James they would also face a wild Sirius (the irony of that always hurt later), and Lily would give him that soft, understanding smile, and he would wonder, did she know? Did she know his secret? His only comfort was that James definitely didn't, and knowing him, he would never even consider it.
Then they got married. James and Lily, the perfect couple. Sirius was genuinely happy for them. He was best man at their wedding, and despite certain eyewitness reports, Moony, he did not cry when they said 'I Do'. Moony was obviously getting mixed up with happy couple's parents.
Then Sirius became Godfather to their son. Harry James Potter.
Harry. That boy meant everything to Sirius when he was born. No one ever said it out loud, but he was more than just a Godfather to Harry. Sirius was like a second father to Harry and Sirius loved him with everything he had. When the prophecy was made Sirius gave up seeing his family (his true family, not his family by blood. He hadn't seen them since he had run away), and went into hiding, even, to make sure Harry would be safe. James and Lily as well, of course, but Harry was the centre of his universe. He always had tended to obsess over things. He failed and that failure haunted him for the rest of his life.
That night, October 31st 1981, Sirius' world was destroyed. Everything he lived for was taken away from him in a terrifying, blinding instant. James, Harry and Lily, his family was gone. Dead, taken, lost. Lost to a madman, who believed everyone was inferior to him. Lost to the Muggles, his only relatives, sent to live with those who hated him. Sirius lost his mind, laughed as they took him away, the bodies and blood surrounding him blurred into the background. Little Peter … Wormtail, who he called friend, who he would have died for, would have seen them all dead. His family, those he lived for was dead, his only friends who remained living would believe him to be the traitor. What did he care if they took him to Hell on Earth? He was already there in his mind. For twelve long years he rotted in Azkaban and true madness invaded his mind.
Harry though, for Harry and for revenge Sirius did the impossible and escaped Azkaban. He had been in that place twelve years and never once found a thought strong enough to give him the strength to free himself, but then he had seen the article in the Daily Prophet, seen the photo of Peter Pettigrew, Wormtail the Traitor. He would be going with one of Weasley boys to Hogwarts.
Harry would be in danger.
That was not a happy thought, and you can't have happy thoughts in Azkaban. In fact, it became an obsession. He's at Hogwarts.
And so Sirius escaped, found Harry, claimed Harry as his own, and once again Harry James Potter was his reason for living. His obsession had only grown with his time in Azkaban and now Harry was, quite literally, everything to him.