Why are most dogs so hellbent on, and obsessed with, playing fetch, never getting tired of the game?: originally appeared on Quora: The best answer to any question. Ask a question, get a great answer. Learn from experts and access insider knowledge. You can follow Quora on Twitter, Facebook, and Google+.
There are certain patterns of behavior that can be found in all canids. One is that they will orient toward something they hear, see, or smell. If they can see it and it moves they chase it, catch it, shake and kill it and then shred and eat it. These are not 'learned' behaviors, they come built in.
Your question should more accurately be, why are 'some' dog so hellbent on fetching?
Working dogs were selected in part based on the presence or absence or strength of the behaviors mentioned in the pattern above. Herding dogs orient and look at (eye) and chase. A border collie in the habit of biting and killing is not selected for. Retrievers will grab but are less inclined to shake and eat. We teach them to come back to us. Terriers follow the pattern through to the kill, though I'm not sure if handlers let them shred and eat what they catch.
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. if you have a dog who in their genetic mix happens to be strongly predisposed to chasing and grabbing something, and at some point introduce them to the game of fetch, you can end up with a dog who wants to do it a lot. This level of drive or motivation might have been highly sought after if you were a hunter and didn't want a dog who decided they needed an afternoon nap in the middle of a hunt.
I have a personal theory, and it's just that, I have no evidence to back it up other than anecdotal and that is that a dog who has less going on in their life is more likely to become addicted to whatever activity they do have access to, which makes them feel good. So a dog who spends upwards of 15 hours a day doing nothing, living either inside a house or out in a kennel, gets to do something rewarding for any amount of time, they are more likely to become obsessed with the activity. Also, given that the average healthy dog can walk or run for more hours than they usually are given the opportunity for, you're looking at a lot of energy to go toward doing something else, like fetching balls.