He didn't know where it traced back to. It was one of those things that just happened, you know, like growing up or working on an essay. You don't realize how much bigger something got until you look back and see how much it's grown.
Thinking hard, he guessed that it probably started near the end of third year. Summer, because he remembered an open window and the content laziness only hot weather can bring. It was a bright day. He was sitting behind her; this part he remembers clearly. Part of his vision was obscured because his hand was pushing up the skin of the cheek that was resting on it; light streamed in from the window to his right, and caught on her hair.
The professor's voice was droning, bee-like, in the background. He was more interested in her hair. It glowed blonde under the sunlight and then the sharp shadows made some parts seem almost black.
He remembers it being much more interesting than whatever the professor was trying to make him listen to; it was a curly, chaotic mass with odd protruding hairs that fell down into random spirals to just below her shoulders.
He was young then, and stupid(er), and absentmindedly reached out with his free hand to tug on one of said spirals then watch it bounce back into place. He hadn't realized how much that would irk her until she turned around and cast him a very steep frown; it got deeper as he grinned lazily back.
So if you ever asked him, 'when?' -- well, he wouldn't tell you, but he'd think of that day, that point of time way back at the beginning of third year; not everything has a grand beginning.