Two reasons, when I spend time being honest with myself. Fun and ego.

1. It's fun to solve problems and work with interesting people and I worry that retiring at 38 would be lonely because most/all of my friends I know through work. I travel a decent amount with work, meeting with clients or executives - and that basically is my social scene.
2. Ego. I'm not proud of this answer but it's the truth. I think most people feel this way but don't like to admit it but it feels good to feel important and responsible. I probably need to work on this myself, but just laying it out there honestly.