I asked this question of all of my students recently on an online poll. At the end of the day 7% of my students said it was looking cool, 14% said learning something new every day, 38% said the best thing was being able to do something other people can't do, and 50% said that making beautiful music was the best thing about being a pianist.
Have you thought about why you are a pianist? Are the reasons still the same as they were when you first started taking piano lessons? Is it ever a combination of all of those answers?
Some pianists play to entertain others; some to entertain themselves. Some like to share the music they produce and some want to be left alone with it. For some, working through something new at the piano makes them feel really smart and for others, it is all about the finished product.
I admit that the best thing about playing the piano changes all the time for me. Whether I'm a teacher or an entertainer or a philosopher or a therapist; whether I have a musically educated audience, an undiscerning and eager audience or whether I'm playing just for myself - I'm always getting these things out of it. I'm always feeling great about improving every day; I'm always proud of what I've accomplished and I
almost always get to make beautiful music.
Happy Practicing!