As OrangeAppled said, for me, it's hyper self-awareness. I'm very critical of myself for the things I mess up, especially when it comes to speaking since I have an African accent and try to speak in an American accent. For the most part, I'm able to do it, but sometimes the accent breaks or I pronounce a word wrong ('order' is my most hated word ever because of this) and I get flustered, which leads me to messing up even more and I end up embarrassed and ashamed of myself.
We cast away priceless time in dreams, born of imagination, fed upon illusion, and put to death by reality. - Judy Garland
I think because they have been judged too harshly in the past. People feel insecure and most of the time I think it's because of false comments or harsh ones, I'm not sure exactly. It's almost the same with how you view your image I think. Check this site out, I think it has a bit of similarities with weight issues and stuff. http://healthmad.com/weight-loss/weight-issues/
I don't get it, why do people feel ashamed of themselves, hate themselves or feel like they somehow need to do something to validate their existence? what is the psychological root of shame and feelings of worthlessness? they don't make any sense to me.
I think it's how people cope when life or things are not going the way they want them to. This was explained to me by a wise person I know and it made sense. The alternative reaction is to hate others. A person who is unhappy will pick one or the other, either hating themselves or hating others.