Why do I only like guys until they like me back?

I'll start out liking a guy a lot. Then I'll really want him to like me or ask me out. Then the second he starts liking me or asks me out I immediately don't like him anymore. Its almost like I'm repulsed by the thought of them. I know it sounds mean but I'm honestly a really nice person. I just don't understand why I get this way. It happens all the time. I'll really like a guy but then he'll like me and it will be like he disgusts me now. This has happened with several different guys. I've never had a boyfriend for more than a week because of it. And no I don't like girls I'm sure of that.