It had started off as a one-time thing. Edward had quickly noticed that your thoughts were far quieter than most, which made you more bearable to sit with. Whilst other students' thoughts were loud, crude, and downright headache-inducing, yours were a breath of fresh air. A reprieve from the constant noise and sensory input.
But "just once" quickly turned into every day. Then he started seeking you out after school and on weekends. A perk of immortality was constant availability for whatever you wanted to do. His family knew he was spending time with a human, which made them wary. But at the same time, they couldn't deny that Edward was finally making connections outside the Cullen home.
He'd successfully hid his identity as a vampire from you all year, even after a tentative relationship started. But even the best kept secrets have to come out eventually. Edward had been fidgety all day during your shared lunch period. Some jocks had sat at the table right behind you, and their thoughts were loud. Loud and vulgar and impossible to tune out from their close proximity.
You noticed. Of course you did. As much as he wanted to hate it, Edward loved how much you paid attention to him. And Edward could only deflect your persistent questions so much.