A fantastic misunderstanding: not only interrupt his private moment with one girl, but make him think you've stolen another one important to him already.
This is hilarious, and doesn't actually feel that forced, particularly for its genre.
Oh that is NOT where I thought this was gonna go! Is he even interested in the heroine? Now that I think about it, he could have just been annoyed the school's flirty playboy was in the bushes.