As a mental health professional, I hesitate to weigh in on what may be little more than opinions that are resistant to change. However, I would say that the definition of "disease" assumed in the previous posts is somewhat limited. It sounds as though some of the respondents are assuming that "disease" is restricted to physical anomaly or dysfunction. Believe me, there are plenty of diseases that plague the mind and leave the body in tip-top condition. You can find a pretty exhaustive list of them in a chunky volume referred to universally as the DSM-IV.
Do people start indulging in self-destructive behaviors of their own volition? Of course. The question any clinician would ask is "Why?" What drives an individual to seek out an altered mental and/or physical state? It's not because everything is peachy, believe me. Is there a physical (and perhaps even a genetic) predisposition to dependence in some people? That's been my experience, although the jury is still out on the genetic piece.
I spend my days (pretty long ones) trying to help people get out of things like addiction. I try to do it with compassion and without judgment. After all, when I leave my office, I rush home to support my own obsession with pen making.
"Sure," you may say, "but it's not self destructive!" Really? Do you all have a dust collection system?
Whether you define addiction (to drugs, alcohol, food, tobacco, sex, exercise, or the NFL) as disease or personal failure, I hope you approach anyone who isn't everything they're supposed to be (and I think we all fit somewhere on that expansive spectrum) with compassion.
Not preaching, not judging anyone else, just practiced in trying to patch together broken lives.
Doug