My wife says that I've got a theory for everything, and here's one of em.
A good dog will worm their way into our heart, and steal a little part of our soul, our personality, and our love. They become an important part of our lives; a part we wouldn't wish to lose any more than we would a limb, our eyes, or another family member.
When they do leave us - all too soon - it is a terrible loss, one of the worst that we might endure, and it takes us awhile to recover. We learn to live without that part of ourselves, and usually go on to give another part to the next faithful pooch that chooses us.
Then -- someday when we finally cross over to the other side, God makes us whole again, and our faithful old friends return to us in good health, with clear eyes and wagging tails.
This might sound kinda corny to some folks, but I'll bet not to the ones who love a dog. jd