As a human, to just buy him a dirty old tennis ball is just not enough, even though he doesn't care. I feel better because I've given him something more than just a tennis ball.
The house is suddenly quiet and because the pet is replacing the children, you tend to humanize them a little bit. You also want to reward your pet more in terms that make you feel good. It's no longer just enough to find a couple of tennis balls and say, 'You've been a good dog today.