The second fundamental reason is somewhat less noble. Artists are very aware of their ilk. The more deeply they are into art the more deeply they are aware of what others like them think and feel about them as artist. Thus, they paint to impress other artists. I've touched upon this before and in much of the art world today, as in the past, it is a driving force, often (suposedly) in conjunction with the first reason, but in reality, often over-riding it. The juried art show, in fact, practically DEMANDS this consideration.
And finally, the artists likes to eat. I know I do. Boy, do I like to eat. It's why I teach art, as oppose to merely creating it. The third fundamental reason artists paint is to sell. Making money doing what is fun, is fun. Painting to impress other artist is one thing, painting to impress a buying public is another and the two are incredibly different, often tearing the artist in two opposing directions like the rope in a tug-of-war. Now, the gift of contentment as an artist is in balancing these often opposing needs, finding self-expression doing work that satisfies, self, peers, and purchasers.