I don't think you really need that much of a reason to develop a crush on someone. In this case I think he's just the right guy in the right place. All the other guys at school kinda avoid her or are intimidated by her. He's into her as an idol, but he's also respectful and considerate. Also, this girl's just a kid and this is her first love, people don't exactly tend to be super discriminating at this stage.