Several reasons:
1. I have a bit of a heart condition. Not a really serious one, but I want to keep healthy so it doesn't become a serious one.
2. Pick up chicks. You can fairly easily meet women in the gym. And women will be more attracted to you if you don't have noodle arms, a flabby gut, etc.
3. I have arms and legs that look like wet noodles. I don't like looking scrawny. (see reason 2)
4. My dad has diabetes now. I'd like to avoid that.
5. I have a flabby gut. I want a reasonably flat stomach. (again reason 2)
6. I'd like to be in reasonable shape to play some sports.
7. Hot chicks in spandex. (see reason 2)
8. I used to have physical problems with my legs. Which caused me to give up psychologically on anything physical when I was little. I have now adopted the opposite attitude and want to work through any problem I have like that.
9. The hot chicks are running, bouncing, bending, stretching, sweating, etc. (again, see reason 2. yeah, I'm a pervert)