“Humans report a wide range of neurobiological rewards following moderate and intense aerobic activity, popularly referred to as the ‘runner’s high’, which may function to encourage habitual aerobic exercise. Endocannabinoids (eCBs) are endogenous neurotransmitters that appear to play a major role in generating these rewards by activating cannabinoid receptors in brain reward regions during and after exercise…”

Recent work supports direct links between eCB signaling and exercise in humans…

It is possible that neurobiological rewards induced by eCB signaling are an ancient human trait that evolved to encourage aerobic activity, and that the rewards explain the evolution…

The fact that running, and endurance exercise in general, remains an enjoyable and psychologically beneficial recreational activity for tens of millions of humans today suggests that we still may respond to a neurobiological trait that evolved early in our lineage.”

http://jeb.biologists.org/content/215/8/1331.long