Why do boys like to wear girls clothes?

The answer is very simple. Everyone likes to feel pretty. Girls clothes are definitely much prettier than boys clothes. Therefore, a lot of boys will get up the nerve to ignore peer pressure enough to try on girls clothes.

Of course that's just for boys that wear girls clothes, aka cross dressers. There are also transvestites, and transsexuals, each having their own reasons for doing what they do. But that's outside the scope of your question.