Something else that science can tell us about exercise: we're wired to enjoy it. More specifically, humans and other animals with a propensity to exercise produce more endocannabinoids which trigger reward signals in the brain.
Picture by Tony Alter
A study in the Journal of Experimental Biology compared endocannabinoid production in humans and dogs (naturally active mammals) and ferrets (naturally inactive animals). Both humans and dogs had high levels of anandamide (an endocannabinoid) after exercise — a substance which helps produce the so-called "runners' high" — while the ferrets didn't. That suggests that natural selection has favoured the production of anandamide, which in turn makes exercise more rewarding.
Unfortunately, that sensation only occurs after intense exercise. As study author David Raichlen points out: "Inactive people may not be fit enough to hit the exercise intensity that leads to this sort of rewarding sensation." Time to hit that treadmill, I guess.