There’s a strong amount of evidence suggesting that the pain relief people experience while running (runner’s high) is very similar to the pain relief people get from smoking cannabis. Both forms of pain relief come from the effects upon our endocannabinoid system, and may help explain why certain forms of pain are treated better with cannabis. What’s particularly notable is that endocannabinoids reduce sensitivity to pain.
I think it is the endocannabinoid anandamide that is released. Anandamide is a Sanskrit word for bliss.
It is the "bliss molecule ". Also theorized that it is released during breastfeeding.