Because boys are idiots.
Lol I dunno. I've never had any brothers or anything, I didn't know my dad 'till last year, and I get along with my guy-friends pretty well so I've never had much of an experience with this sort of thing.
Not all guys do. Some are kind, helpful and understanding. Others are rude, irritable and insulting. It really depends on who the guy is and what mistake the girl did.