Why are some guys so nervous to ask a girl out?

Okay, so if a guy knows that a girl likes him, and he is still afraid to ask her out, why is he so darn nervous? Is it that slight fear of rejection? The guy i am thinking of has really really bad facial tics (he blinks like crazy), he is NOT athletic, and he is kinda a geek. Despite being a geek, he has trouble in school and some learning difficulties. He is kind of rich. We have discussed getting together, and he has made it clear that he wants to sometime. Some guys think i am a babe (not to be conceited). I wear juicy couture, victoria's secret PINK, tiffany's, PUMA, david yurman, etc. Guys that have crushes on me (and my dad too) mimic my valley girl voice. His cousin said that this hott geeky guy really likes me and wants to ask me out, but is extremely nervous (WHY)?