I personally do not enjoy UI development because I am not good at it. There is HUGE element of user psychology that I am just not good at understanding. I think my biggest issue is that I can't put put myself in the user's shoes. I do not know how to make intuitive layouts largely because I do not know what is intuitive to the user, nor do I know how to make things look pretty.

I don't necessarily think some programmer hate designing UI's as much as they hate doing things that they are not good at. It just happens that there are a lot of developers that are not good at UI development.