I think this is a great thread topic and have been wondering about similar things over the past little while. I've been invited by a friend of my wife's to join their little art show/sale in early December (they are a group of three to five painters that host a show/sale twice a year). At first, I thought 'cool...awesome'. But, then I started thinking about some of the things you mentioned. Flipping through my photographs it really sunk in that I am doing this for myself. There isn't really a comprehensive vision, just a bunch of stuff I am interested in.
I really like some of my images and the few people I've shared them with seem to really like them too, but I don't really have a clear vision for this work of mine. I'm struggling with what 'art' is and if I'm even making it. I never really considered selling my work, but with that potential looming, I find myself asking a lot of those philosophical questions.