What is it about playing tennis that makes it fun? Is it necessarily winning? If not, what else is there?
Connecting with the ball; delivering a beautiful serve; getting the ball to go exactly where you wanted it to go--I'm sure the list could go on.
It's not unlike playing the piano for pleasure. Having to win is like competing with other players in terms of numbers of notes, speed, perhaps repertoire. For non-competitive types there are other sources of satisfaction among which savoring dissonances ranks high on the list.
One of my adult students pointed out that dissonance has an entirely different impact in Bartok than in Mozart. Learning to savor this has brought both maturity and satisfaction to her playing.