For me, the fastest way to scare off a girl is simply being myself. I'm that guy that every girl says they want to date--"I want someone who's sweet, nice, caring, pays attention to me, don't have to worry about him cheating on me, etc..."--but as soon as any girl gets to know me, "Oh, you're a good friend, but I don't think we have enough in common/you're like a brother to me/you're too nice/you have plenty of great things to offer a girl, but not me." It's gotten to the point where, if I girl complains to me about not being able to find the right guy, I have to fight the urge to yell. Sometimes, I lose the fight.