Why do I think that people are only nice because they want something? Please help me be less cynical!?

This has been a big problem for me since a BAD relationship about 3 years ago. It completely destroyed my trust in all people; not just men. Since then I've been trying to figure out why I feel so cynical about people who are nice to me. I'm a very modest person so I don't feel like I deserve attention as I'm no different from anyone else. Usually when people are nice I ume it's because they want something from me. I feel no need in being friendly, having friends, ociating with co-workers on a personal level (even though they desperately try to make friends) I always feel as though I have nothing in common with them what-so-ever so what is the point in trying to make a friendship that will not last. It has become a major problem because my fiance is getting fed-up with my self defeatist, negative and moody personality. I never used to be like this...and I'm starting to hate myself. Can anyone tell me how to fix this? PLEASE BE HONEST!