I went on a community music "Group dynamics" course run by Music Leader a couple of years ago. One of the main things that stuck were ideas on what motivates people within a group, i.e.
belonging - competence - autonomy
So feeling motivated within a group is easiest when:
- you feel like you belong, there is a place for you
- you feel that you have something to give, that your skills are valued
- you feel that you have some choice over what to give, you're in charge of you
I feel that folk sessions fit this exactly. At a session, where you met voluntarily in the pub, if you turn up with an instrument you can become part of the session - belonging. Part of the way folk music works is you can subtly alter the music with ornamentation. This means that many people can play the same tune while all playing at different skill levels and all having responsibility over their own part - competence and autonomy.
I feel that all improvised music allows this balance - that you are playing with what you have, not what you don't. For me, this is a defining feature of social music - that it welcomes all without constraining any.