From my time playing and organizing tennis at many places, I have noticed that those who play, share a common experience, which includes a particular passion. I think it has something to do with the unique scoring system and the fact that as a tennis player you are out there on the court with your opponent(s) on an honor system, calling lines and keeping score. You have to admit to good shots and accept that a ball that hits the outside of your line means you lose the point, and potentially a game, set or match. It also has to do with learning, after awhile, that there are so many ways to hit winners and, much more often, to miss shots, that you never really knows what will happen or who will win the (last) point.
There is always something to talk about after a game of tennis at any level! I think an understanding of this general tennis reality creates a sort of inner smile in people which helps foster a unique camaraderie, both on and off the court.