Why are guys so scared of women who are "pretty, good girls"?

Just my personal opinion, I see the same happen all the time, and i believe it is a issue of self confidence, and a paranoia that they have to make a good impression on the gorgeous woman, and not being comfortable with the person that they are and the try to be more like the type of person they think the girl would be into. But who would spend the time trying to be some one their not am i correct?