I really love "watching" the brain in action: learning, accepting the data into its files, and integrating. Practising the piano (or any instrument, I suppose) gives one such clear feedback on where exactly in the learning process one is. If the brain is not carefully programmed, the physical tasks of playing are not successful and signals abound: wrong notes, clumsiness, and just unsatisfying, unmusical playing. The emotional signals of incomplete preparation and learning are also quite clear: doubt and anxiety, which of course can lead to complete DISintegration in performance.
When the programming of the brain (otherwise known as learning) has been done in a thorough and effective way, the body responds with gestures of ease and elegance. These two attributes are always my goal when practising.