Here's my best guess. It's like a bell curve and we, the audience, are the majority. At one small end are the messed up actors, and the other end are the competent and learned performers. You have your druggies plus your people playing at very high levels and this one thing unites them: they can show us publicly what we do privately.
I didn't invent that. Rob Reece (my method acting teacher in SF, now in Bev Hills) once said, "An actor is one willing to do in public what everyone does in private." He probably got it from Stanislavsky.
It takes a special person to portray intimate feelings for an audience. One exercise Rob subjected me to--and there were palm sweating, heart racing others to be sure--was to have me dialogue out of my butt. Here I am, bent over, using my hands to move my "cheeks" and address the class. Great. Just what every introverted writer dreams about. But, that's just what I needed to get me out of my shell. Nowadays, there aren't many public arenas that give me the willies. Thanks, Rob.
So, people not only willing to show us humanity at its most raw and naked, but wanting and desiring to do it are a special and different breed indeed. Some of them are driven to distraction (drugs, alcohol, sex), and some of them are driven to the bookcase. The rest of us watch.