Runners High and Evolution

The pleasurable feeling triggered by aerobic exercise known as 'runner's high' may have played a role in the evolution of our ability to run long distances. The chemicals that are responsible for this feeling are known as endocannabinoids, and exist in Humans and Dogs, as we are built for endurance. They do not exist in ferrets however and other such species that are sedentary species. The researchers expressed that "These results suggest that natural selection may have been motivation higher-rather than low-intensity activities in groups of mammals that evolved to engage in these types of aerobic activities."

http://phys.org/news/2012-05-evolution-runner-high.html