I went to my comic shop the other day, and there was a guy thumbing through the long boxes of old comics, talking out loud to himself. Mostly he was reciting the titles of comics as he thumbed past them, occasionally commenting on whether he had a particular comic, occasionally bursting into snatches of song (that Beck song about being a loser seemed to be a favorite. A confession?). This wasn't a quiet mutter; this was gabbing out loud like he wanted to be heard. Yeah, a misfit in a comic store; who could have seen that coming?
And a few months ago I was at a bookstore, where I bumped into a clutch of old acquaintances. We started gabbing happily. There was a guy nearby, all by himself, looking at us. Then he went back to looking at books, and proceeded to talk loudly to himself, occasionally stealing glances at us.
I suspect that when you don't know how to talk to people, talking near them seems like a compromise worth trying.