Every time he dreamed at Hogwarts, she was there.
Her red hair flying, her emerald eyes sparkling, her laughter ringing out in peals...no matter what, she was there.
In his dreams, there were smiles, and friendship, and sometimes there was love.
He knew it was ridiculous, of course. In reality, she couldn't stand him, greeted him only with coldness and ire. In real life, she'd never like him.
He guessed that was why he dreamed.