Why we're motivated to Exercise

I just read an interesting article about exercis motivation on the New York Times Well Blog.

Essentially, scientist at the University of Missouri wanted to see if there were genetic variances in lab rats when it came to exercising.

Researchers created their own avid runners and couch potatoes by breeding male and female rats that had a propensity to run.  They bred the male and female runners together' and the couch potatoes together through 10 generations.  At that point, the running rats tended to spontaneously exercise 10 times as much as the physically lazier animals.

They looked at the bodies of the rats, to determine if there were physiological differences between the active versus sedentary animals.  Aside from the lazy rats being slightly heavier, the rats were essentially equal.  The running rats seemed to enjoy the activity, while the lazy rats avoided it at all costs.

This lead researchers to look at genetics, and here, researchers found differences.

So what does this mean for us humans?  Are some of us genetically pre-disposed to exercising (and enjoying it) while others are not?  Can some of us use genetics as an excuse not to exercise?

Using myself as an example, my parents were/are not particularly athletic or active people, so where did I get my passion for running?  My son, doesn't seem to have the motivation to want to exercise, so it seems that there must be some other factors to look for...