Sometimes I play something on the piano so much that after a while it's just... routine. It's not as exciting as the first time I played it, or even heard it. I think part of what makes music appreciable is the surprise factor. If you know exactly what the whole thing is going to sound like, then you're really just waiting for it to be over. The compositions unique chord progressions and melody lines have long since worn out their impression.
Well, I find that if I transpose my keyboard and play the piece in a different key, it's like hearing it for the first time again. Even just a different of a half step, and I'm giving myself goosebumps all over again.