It's definitely a very interesting phenomenon. I'm from nature's side extremely introverted (but not "shy," never really was) and my nervous system tends to be very highly... sensitive... to environmental stimulation. I experience a lot of "vigorous" things most people find stimulating and fun to be overwhelming and outright painful.

I've read "Attached;" definitely a good read on attachment styles, and understanding where they come from; I learned mostly from a disorganized/chaotic model, leading to extreme self-reliance as a teen and beyond.

In general, the avoidance I have dealt with has been more experiential/environmental than interpersonal; my friendships and relationships have mostly been pretty stable and long lasting.