This sounds corny, but health is the key to happiness. You can't enjoy many of life's pleasures if you don't have your health. I guess I think of exercise as system maintenance. I know I need to stay active to manage the stress in my life so that I can be a more productive, energetic person.
Nice weather is also a great incentive because there are many fun activities to do outside.
And having friends who are physically fit and active motivates me because we plan activities together. My mother is my inspiration. She looks 10 years younger than most women her age. I hope to follow in her footsteps. I rent books on tape and discipline myself to only listen to them when I'm walking.