Because of runners’ high:

When people exercise aerobically, their bodies can actually make drugs — cannabinoids, the same kind of chemicals in marijuana. Raichlen wondered if other distance-running animals also produced those drugs. If so, maybe runner’s high is not some peculiar thing with humans. Maybe it’s an evolutionary payoff for doing something hard and painful, that also helps them survive better, be healthier, hunt better or have more offspring.

So he put dogs — also distance runners — on a treadmill. Also ferrets, but ferrets are not long-distance runners. The dogs produced the drug, but the ferrets did not. Says Raichlen: “It suggests some level of aerobic exercise was encouraged by natural selection, and it may be fairly deep in our evolutionary roots.”

The story is from NPR, the pointer is from Balazs Szentes.