Fido loves playing fetch, he really does, but our canine besties partake in this pastime for one main reason: we bred them that way.
When humans were first domesticating dogs, one of their main jobs was to aid in hunting and retrieving food. Some pups were better at this than others, and those who excelled would have been bred to keep the trait around.
Today, some dogs are maniacs for the game—retrievers and spaniels, for example—while others have no interest, either by breed or personality. But if a dog is genetically predisposed to the activity, it might not take much to get him or her hooked on fetch.
As Debbie Jacobs, author of A Guide To Living With & Training A Fearful Dog, writes: “All of these behaviors are self-reinforcing, meaning they make the dog feel good. They don't need to be rewarded for the behavior. If you like playing football, you play football even if you don't get paid to do it. It just feels good to do it. Same is true for dogs.”
On a chemical level, dogs who love fetch experience the same thing us humans do when we exercise—what’s often referred to as a “runner’s high.” Their brain releases neurotransmitters that tickle reward regions and elevate their disposition.
Above all else, dogs are getting what they want the most: undivided attention. Verbal and physical rewards, and maybe even a playful roll in the grass with their human companion, are added bonuses.