When you know a piece and play it, it can be fun. But the way there can be frustrating and tedious. You'll make mistakes. You'll learn slower than you'd like. You might repeat the same section a hundred times before you feel satisfied. Some days you'll even feel like you've forgotten how to play certain parts properly. But you do it anyways because you know it's rewarding and satisfying at the end of the day (and with a certain level of discipline), not because it's fun moment-to-moment.