I often lift weights, because for one I am a athlete but also it helps me stay healthy, and feel good about myself. Tons of people look at themselves in the mirror every morning and are dissapointed in what they look like. I have vowed to never have that happen to me, i'm not a crazy gym junky that goes to the gym at 6am and comes back at 6pm, but I do lift enough to keep me in good health and to have me be proud of my body whenever I go to the beach.
We seem to have a similar mentality towards this. The other thing is that when you get into good shape (good is relative, but whatever the peak is) it doesn't feel nearly as good when I'm (relatively) out of shape.
When I compare how I feel between those two states, life is way better when I'm in good shape. We all have to drag around this bag of skin our whole life...might as well take care of it...and push it to the limit every now and then for the thrill