Severus was angry and embarrassed. He wasn't thinking. Potter and his "friends" - groupies, more like – were torturing him in front of everyone. Again. Severus had had it. He was more than ready to take on Potter, Black, Lupin, Pettigrew, and everyone else in that courtyard. Then, Lily Evans came in to defend him, and all his stupid, moronic fool of a 15-year-old self could think to do was insult her and make his life even more miserable. You see, Severus may have been a Slytherin, and Lily a Muggle-born, but love is blind, deaf, mute, and insane, and though he wouldn't admit it, even to himself, Severus Snape was in love with Lily Evans.
He and Lily had been friends for years. When they were first sorted, he almost wished to be in Gryffindor, just to be with her. Through the turbulent school years, they had stayed friends, despite House rivalry and each of their friends' utter dislike for the other. But this, this was most inexcusable. He was never going to win her back over, and he knew it. But he would try anyway.
She came out of the portrait hole. He stood up immediately.
"I'm sorry."
"I'm not interested."
"I'm sorry!"
"Save your breath. I only came out because Mary told me you were threatening to sleep here."
"I was. I would have done. I never mean to call you Mudblood, it just -" He was desperate by this point. He knew, he knew this meant she would never forgive him, even a little bit.
"Slipped out?" Lily interrupted. There was no pity in her voice at all. "It's too late. I've made excuses for you for years. None of my friends can understand why I even talk to you. You and your precious little Death Eater friends – see, you don't even deny it! You don't even deny that's what you're all aiming to be! You can't wait to join You-Know-Who, can you?" He opened his mouth, then closed it without speaking. Every word was breaking his heart, piercing it like arrows pierce a dummy in the training field. "I can't pretend anymore," she continued. "You've chosen your way, I've chosen mine."
"No – listen – I didn't mean -"
"To call me Mudblood? But you call everyone of my birth Mudblood, Severus. Why should I be any different?" He wanted so badly to say something, anything, but the words wouldn't – couldn't – come. With a last look of contempt, she spun on her heel and went back into the Gryffindor common room.
It's been 15 years since then, perhaps a little more, perhaps a little less. Lily hasn't spoken to him since that night, so long ago. I think that's what finally turned him. Lily was also what brought him back, just a few days ago. He went to Dumbledore, begging forgiveness. Lily, the girl he had been in love with since he was but nine, and at twenty-nine still is, was in deep and terrible danger, and he would do anything, anything, to save her. By then, of course, it was already far too late. Despite the Order's best efforts, the Potters were murdered tonight – all but the infant boy. Severus' emotions are strong, turbulent, and most likely confusing, on the subject of young Harry Potter. The son of his worst enemy, the sole continuation of the love of his life. Only time will tell how he reacts to Harry when he comes to Hogwarts.