To say that "pretty girls" only say they're ugly to hear the oposite is of course a terrible generalization. But I also think that it can be a way some use to get attention or to feel better about themselves.

I'd also like to mention that there is a compulsive disorder called BDD (body dysmorphic disorder), people who in other eyes look normal but they still think that they are incredibly ugly or defected in ways. I think I read somewhere that it is almost as common among men as women though.

I would write more on this but I don't have much time right now, I'll come back later :P