Disclaimer: Still not mine.
Sorting
It was time.
The severe looking professor had led them into the Great Hall to the stool on which a raggedy old hat was sitting. But the unremarkable appearance of the Hat belied its power and it suddenly burst into song. And because he didn't know what to listen for the only thing he really heard was the Hat's description of Slytherin: cunning, ambitious, and thirsting for power.
Yes, that's what I want! I want to become powerful enough to protect Lily! Powerful enough to protect my mother from that filthy Muggle who calls himself my father! I can do it if I'm in Slytherin and Lily can help me! But while he was busy with such thoughts Lily had moved to the front of the line and focused on the Hat with renewed intensity.
Slytherin, please Slytherin! But then the Hat shouted "Gryffindor!" and his hopes turned to dust.
Maybe, once I learn, I can transfigure the insignia on mum's old robes. I can be in Gryffindor, I don't need to be in Slytherin as long as I'm with her! And all too soon it was his turn to put on the Hat. Please, please, put me in Gryffindor! I need to be with Lily, she's the only friend I have! I can be a Gryffindor, I swear!
"Gryffindor, hmm? I'm not so sure you have the heart of a Gryffindor my lad; you seem to be one of the Slytherins. You could do very well in Slytherin you know, and there are friends to be made there. Powerful friends who can help you reach your goals."
No, you stupid hat! I don't care about any of that unless she's with me! I don't care about power, or being important, I just care about her!
"Then I'm sorry. But you don't have the heart of a Gryffindor yet, and without that I can't place you in that house. And as you're unfit for Hufflepuff and far too cunning to be in Ravenclaw, the only place I can put you is SLYTHERIN!"
And that was that. It was over. He could never be with her again, at least not in the way that he had grown accustomed to. But maybe she wouldn't mind that, and maybe they could still be together. Maybe.