A great snippet from the NY Times on the evolution of the runner's high:
"Liking to run, it seems, may have helped to make humans what they are...our evolutionary history appears to have included this kind of endurance activity and rewarded it. And as a result, we continue to have a biological imperative to move."
This reminded me of the book Why We Run, an easy-to-read, fascinating biological perspective of how humans have survived and evolved because of running. Our ancestors were able to hunt by out-running animals who have fewer sweat glands and as a result, significantly less endurance capacity. Humans also evolved with physiological traits to enable efficient running, such as the stretchy nuchal ligament in our neck, long Achilles tendon and plantar arch in our foot.