You know, in all the ways you can categorize people who are mentally screwed up...
[Dx: dysthymia/aspergers/AvPD/social phobia/psychotic depressive episodes]
I think a lot of times, half? More? The "screwed up" is, to broaden the label, some form or another of "Arrested Development."
And I think when people relate to each other and form bonds, especially when we bond over commiseration and pain, what we're really doing is finding others whose growth got impeded at the same stage in our lives. I think this bond crosses not just a lot of mental Dx lines, but a lot of different social lines as well.
We get ###$ up at different times in our lives. Sometimes in pregnancy or infanthood even. Sometimes mostly at one critical age, sometimes at multiple ages. And each time, some potential within ourselves gets stopped. Not all painful experiences do this warping/inhibiting. And sometimes the things that ###$ up us the most aren't the most painful, they might not even hurt at all.