Actually, I think the two need not even be related. As in, we can be shy/anxious/nervous/insecure/whatever, whether we stutter or not. It's not necessarily a "function" of stuttering. Of course, stuttering may factor in, with regards to self-image. But I think some people are just shoving all their shortcomings together, and believe them to be connected. I imagine it must be hard to try to develop out of it, in that case.