The following is an excerpt from the book "Old Dog" by Gene Weingarten and Michael S. Williamson. It is a nice analogy of dogs and humans as they age:
In our dogs, we see ourselves. Dogs exhibit almost all of our emotions; if you think a dog cannot register envy or pity or pride or melancholia, you have never lived with one for any length of time. What dogs lack is our ability to dissimulate. They wear their emotions nakedly, and so, in watching them, we see ourselves as we would be if we were stripped of posture and pretense. Their innocence is enormously appealing. When we watch a dog progress from puppyhood to old age, we are watching our own lives in microcosm. Our dogs become old, frail, crotchety, and vulnerable, just as Grandma did, just as we surely will, come the day. When we grieve for them, we grieve for ourselves.