While a person may be motivated to begin an exercise routine in order to lose weight or improve health, the drive to exercise often wanes over time.
There are no good studies investigating why people keep exercising. A NYT article by Gina Kolata examines the motivation to continue exercising. She finds that many people continue to exercise due to the feeling they get from exercise.
It appears that a desire to improve health is not a long-term motivator to keep exercising. The feeling of being energized, improved mood, and feeling restless without exercise appear to be the motivating factors for long-term or life-long exercise.
More study is necessary, but these findings may have implications for public health programs and wellness programs promoting fitness.
See the article here.