I just had a game of tennis. Chatting with my tennis buddy afterwards we were talking about why we prefer playing tennis or any sport over just going for a run. The main reason for both of us is that sooner or later running, for us, just feels like work.
We're not runners you see. I know I can run 6 or 7 miles relatively comfortably but I just won't enjoy covering that distance half as much as covering the same distance on a tennis court or a football pitch. We also know that we won't put in the same intensity (quality) or effort. In the end your body simply adapts to the demands you place on it so we get a much better workout from a simple game of tennis than we ever do from a run.
The main factor is that we enjoy it. So we want to push ourselves. It takes so much longer on the tennis court for us to feel so tired we need to stop. We've both learnt that once something starts to feel like work then we can't class it as fun and we need to change it. It's also true that I owe so much of my health to tennis because I want to play come rain or shine, in the hot and the cold. it keeps me active in all weathers and no matter what my work load. Without it I'd succumb to stress and I wouldn't want to be active.
So ultimately it's the fun in the activity that keeps me going.