The emotion of the music is almost enough to keep me at it in and of itself. However, I've always had this drive to perform my music for other people and that automatically sets a goal when I take up a new piece. I'm not saying it has to be on stage someplace ( though I have done that in the past) but even in my living room, the goal of the new piece is to play it for myself and for someone else as well. I put together little performances, I may play a couple of pieces from it or the entire plan when someone visits, for instance. I keep people in mind when I select a piece to learn, one that will be nice for an individual or group of people to hear and that I will enjoy working on. That whole way of working will always keep a few pieces up and running even if they are a work in progress.
The piano is pretty big in my life, I maintain my own grand piano, I study and learn new music to me and I have a couple of students. So the piano keeps me pretty busy. I'm either rehashing theory and presentation for a lesson for one of the students ( I am not a professional teacher, I only teach people who ask for lessons from me, be that family, friends or aquaintances), physically working on the piano or working on my own pieces at the piano.