This happens to me, too. In my case, I think it's self-sufficiency. Some people don't really know what to think of self-sufficient people. They seem to think friendship is supposed to be... I don't know, involving some level of need I just don't feel. When they realize I don't need them (in some cases, it's been clear they do need me), they distance themselves.

I never have this problem with people who are as self-sufficient as I am.