Playing music or listening to music is escapism, pure and simple. Listening to someone elses music you are transported into their story and their emotions. It is a similar experience to reading a novel or watching a film and identifying with a character. When performing, you have a purely selfish moment in which you can express yourself to an audience and that moment is all about you. What you feel. What you want to say. That is not generally an acceptable behaviour in society (unless you're in counselling) as relationships and friendships demand reciprocation but as performance or art it is fully accepted and indeed expected. In both cases emotional reactions will stimulate a chemical reaction within the brain which you could class as self medication but on a very natural scale. I personally think it is probably much healthier than ACTUAL medication and believe that art/sport/music/conversation are much underated as therapy. People in general would be much happier with more activity and less prozac!