I had a therapist who specialized in AODA (even though that's not one of my problems). Her dad was an alcoholic and she chose to work in a field that could help her cope with that while helping other people. She writes papers and does research on the subject, while being a therapist for people with all sorts of problems, not just alcohol and drugs.

My pdoc (the one I'm leaving) specializes in post-stroke depression. It's usually a combination of their research interest and their personal issues, at least in my experience.

Candy