And I think it is about fulfillment, which is not always about fun and is not always (or perhaps almost never) "about" self.
There is always philosophy, it seems.
Fulfilment is about self, me thinks. Nobody requires anyone to fulfill or achieve anything, it's all about free will. Anyway, personally I don't really care about "fulfillment", it's too relative a notion, and piano or any other form of artistic activity is an endless quest, you never really reach "fulfillment". Fun is definitely subjective but it's absolute, you have fun or you don't, that's it. On the other hand, the click (more exactly the click
s) which allow you to have one piece learned is mostly, even totally, linked to self-awareness. Between the very moment when you start learning a piece and that one other moment when you got that piece mastered to a satisfying level, what you do is overcoming blocks, sometimes physical but most often mental. The only way you can overcome blocks is to have the required level of self awareness which allows you to assess properly the nature of the block and its solution.
Obviously there is philosophy in art but I think not really in the aspect of learning and executing methods, which was mostly implied in the initial question. Now if we talk about the underlying metaphysics of why and how we love (or don't love) listening and/or playing music, then there are indeed as many philosophies as there are pianists and/or listeners.
(OK I'm into semantics here).