Pretty people know they ain't got nothing going for them besides their prettyness, so they know that the one who is into them, if he is into them just for the prettyness and not the personality, then there can very well be someone else who is even prettier than them. Either that, or that they will get old and get ugly.
They know how pretty they are. They need a reason to talk about it especially when noone compliments or finds it worth a conversation.