For many years it has been thought that the "runner's high" comes from the endorphins in the human body produced by the endorphin system. The endorphin system is the system of receptors that opioids activate to help manage acute pain.
However, based on recent studies, it is now believed that the runner's high is due to a rush of endocannabinoids produced by the human body when a certain heart rate level is achieved and maintained for a certain period of time.
This is a good example of how the ECS acts as a reward system for proper activation. As the body exercises, it is rewarded with a feeling of bliss. This is the body's way of reinforcing said behavior.