Because they have low self-esteem.
I believe it's frustration/anger..no matter what they do they never feel improved. They need constant change to get satisfaction in the mirror.Even beautiful people can have these beliefs. Very sad and some die from it.
Because: They want the same thing as everyone else, but always find themselves out of the room.
Who can they blame if no one is around?
Ugliness doesn't make anyone a masochist .... maybe it does if you always want to be punished if it's your fault. My worry is that it does make "ugly" ones a sitting duck for the first predator who shows some interest
Consider the culture... advertising in particular. Have you ever seen a commercial laced with half-dressed people, and that has NOTHING to do with the product?
People think they are ugly not only by face appearance but also body. They want that models body they saw on TV the other day, or they think they are too big and want to lose weight or at times people tell them that they are ugly and they believe but in my eyes no one is ugly and people shouldn't punish themselves if they think that they are ugly, but acceptance is a hard thing to do and also because attractive people are always shown on TV and we tend to compare ourselves to them :P
Low self-esteem, they will treated their spouses as mean as can be because ironically they want them to stay....I've known several gorgeous women who had some homely husbands and it seemed to be a pattern in their marriage.