Exercise in ancient Greece, for example, was an ingrained part of the religion and culture. The Gymnasium was dedicated to physical perfection, but also developed into a meeting place of philosophers and educators.
Sometimes, it's just about pride. The thought of having to buy new clothes to make room for more of you is just not pleasant (especially when you see the selection available). In fact, some studies have demonstrated that monetary incentives have helped people lose weight, such as offering free healthy meals.
At the end of the day, the best motivator for exercise is going to come down to your desire to always be active. If you want to be able to get to many of the places that you saw at 25 later at 65, then the activity level has to be at or near the level it was at age 25. Muscle responds to stimuli. Do nothing, and it goes away. Do something, and the muscle stays with you for many years to come. Having an exercise program that addresses the needs of functional fitness is the best program for everyone.
To learn more about functional fitness and daily activity regimens, please visit ACSM's new recommendations on quantity and quality of exercise