Because for a guy that's socially akward, but very observant, how can he not realize that the confession was off? That she's acting not as happy as she should be? That guy always watches her and can read her pretty well. Heck, she fainted and smelt strongly like alcohol at the confession. I'd love it if he's aware and not letting her come clean in order to become closer to her. Would kill the whole questionable "she's playing with his feelings" aspect, too.