With most manga I'd be annoyed at this kind of side flirting with someone in a main couple . . . but with this I just gave up long ago. Everybody flirts with everybody, they're all cute, it is what it is, whatever.
@KitsuKyouno Because in this case they reference the idea of boyfriend characteristics rather than girlfriend. As such they're essentially saying "You have the traits I wished a boyfriend would have" which is a common trope