Why do people attach the way they do?

One problem I have: letting go. I know it has a lot to do with my past of never making close friends, people constantly leaving my life and I'd subconsciously push people away. I'm fiercely independent until someone proves I should break my guard down. Once the guard is down and they leave me and I'm left hurt and attached. I'll overanalyze as to what I did to the point it depresses me. Then, I can't let go.

This especially happens with guys. They are making it more harder for me to see them in a good light.

There are so few people who understand me. I realize I intimidate people and know I'm "too much" to handle (I've been told this and known this since I was a child). So, if someone takes the time to see who I am, they must be important, right? I'm starting to find more and more that seclusion and isolation are the best ways to keep from getting hurt.

Why is this? And how does someone guard themselves from it ever happening again?

Serious answers, please. Thanks.