People are wired to run, according to a new study that looked at the roots of the pleasurable sensation known as runner's high.Experienced during moderate to intense aerobic exercise, runner's high occurs when natural chemicals called endocannabinoids activate the part of the brain associated with "feeling good," said study co-author Greg Gerdeman."Endocannabinoids are molecules that are often referred to as the body's own marijuana-like substances, because they activate similar cellular receptors," said Gerdeman, a biologist at Eckerd College in St. Petersburg, Florida.