It's a great question. I think the variety of the game really draws me in, there are just so many ways to play and I don't think I've ever played two matches in the same way. Physically speaking there's just so much you can do with a tennis ball, I can't think of any other sport where you need such an in-depth, yet almost entirely subconscious, knowledge of physics.
The only downside for me is that height is such an advantage! It pains me to see players with poor skills and technique playing the game to a high standard simply because they can effortlessly throw down huge first serves and dish out big forehands. I'm 5'10" so it's more of a challenge for me to hit aces!
Yes, the height is sometimes a disappointment. I'm 6'0" but I sometimes do wish I was another 3 or 4 inches, haha. Some people claim "well at least you get good footwork being shorter" but come on, look at Murray/Monfils who are 6'3"/6'4" and they have amazing footwork, footspeed and big first serves!