I think they can be happy because they have a better self esteem, so they know that even if they are not the best looking people, others can like them even so because they are good persons with great personalities... Maybe it is the same for you, but I don't think I am a very good person, and though I know some aspects of my personality can be interesting, I think I am overall unpleasant and boring. That is why I considere I should be amazing looking to allow myself to be in other's company... So at least they would have something pleasant to look at. I am aware of how stupid it sounds...