I often feel that way. I'll meet a guy, get to know him and get a crush on him, but always think, "he's way too good for me" or "I'm not pretty enough for a guy like him" etc. etc. etc.
I know guys want girls to start making the first move, but in my experience, guys are often a little oblivious... They don't really get flirting half the time and for me it's way too forward to be like "I like you. Let's go on a date."
Also, based on watching some of my guy friends get hit on by girls, some guys can be really harsh, shallow and so mean. So it scared me off when it comes to hitting on guys. I realize some girls can also be all those things as well, which is probably what scares some of you guys off. But yeah... I don't deal with rejection well.