Well, you have a pretty low sense of self esteem. That puts in a group including about 80% of the human race. You wanting to push people away suggests you've had some rejection issues with people before and you're afraid of that happening again. That's pretty natural.
I suspect the real reason for your post, though, isn't to understand this feeling, but to change it. You want to be able to feel good about being close to people. If this is true, then you need to acknowledge the fact. This situation won't change quickly, but will one way or another if you hang in there. It's good that you wrote this forum post. Writing out your thoughts helps you get thing straight in your head, even if you don't get a satisfactory answer.
Lyric ~