In the very beginning it was because of the first impression. Their initial encounter on the train had been ghastly. He and his little friend had been terribly rude both to her and her friend, and she had not stood for it. It didn't matter that she had been upset with Severus at the time; he had been the only friendly face on the train and these two insufferable brats had known him for five minutes and were already calling him names. It left a sour taste in her mouth, and every time she recalled the scene left her in a huff.
Later on, she learned more about this grinning bespectacled boy who, by a stroke of ill luck, had become her Housemate. Not only was his blood pure, it was also old and prominent. His parents were both well-known and well-liked, and he knew everyone who was anyone. By all accounts, he was an obnoxious little rich boy, with whom Lily, the muggleborn girl with no standing in the wizarding world, had absolutely nothing in common.
In the second year, it was because for some reason the arrogant git had singled her out. He and his little posse had chosen her to be the butt of their jokes and the victim of their pranks. She would like to think that she had a good sense of humour, but constantly being laughed at, having her buttons pushed, and her hair pulled set her teeth on the edge. Her rather volatile temper did not really help to deter them, rather the opposite. Long story short, even though she paid him back in full for every prank, taunt and hex, he was making her life hell.
In the third year, it was because of his ceaseless talk of Quidditch. Always boasting and bragging about his prowess on a broomstick drove her up the wall. He was rather good at the game, she could not deny such obvious natural talent, but any admiration that may have sparked from his show of athleticism was quickly killed the moment he opened his big mouth. And as if that wasn't enough, he had picked up this infuriating gesture, and it felt like every time she as much as glanced his way, he was mussing up his already messy dark hair. So ridiculous!
In their fourth year it was because of the endless mischief. He and his pals were racking up detentions, always causing trouble, pulling pranks and the like. According to the castle gossip there was sneaking about, breaking curfew, trips to the Forbidden Forest, duelling in the corridors. And to add insult to injury, everyone seemed to find this reckless rule breaking "cool". Even so, to Lily the most aggravating part was that in spite of the marauding and detentions and the fact that she had never actually seen him in the library or studying in the common room, he was brilliant in most of their classes.
In their fifth year it was because of all the tensions running high. Lily was already stressed enough because of her new prefect duties and the academic pressure of the O.W.L year, but to add to all that, her relationship with Severus was starting to grow seriously strained. But of course, even all that was not enough, because he had decided that he fancied her. Every time there would be a Hogsmeade weekend ahead, he would inevitably block her path in the corridor and plead her to go out with him. She told him no, time after time, but he wasn't deterred and it was making Severus more and more sour. It was all heading for a disaster.
In their sixth year her resistance was slowly starting crumble. He had grown almost a foot over the summer holidays, and suddenly she noticed all those muscles Quidditch had bestowed to him. He had also been made the team's captain in some fit of lunacy, but from what she heard from her friends, he was actually proving to be excellent at the job. Still, the disaster in which last year had inevitably culminated had left her in a bad place. And although his appearance was starting to be rather pleasing to her, his personality still left a lot to be desired.
And then early into their seventh year there suddenly came a moment when she looked at him and realised that all of her reasons were gone. Her gaze travelled from the Head Boy badge up to lock with his hazel eyes.
She gave him a smile then, a brilliant one from the heart, and breathed: "Yes."