This could lead to interesting discussion. >Continue?>Y
I think that this could be due to the fact that as humans, we all know that mankind is flawed. Therefore, in some way, however miniscule, there is always some degree of justification for the death of a human. We perceive animals differently, obviously. Animals, particularly animals we are attached to as a species - dogs, cats, some of the other prominent mammals - are viewed as flawless, and without sin, for lack of a better word. And so, all animals, and particularly dogs, I think, have an association with purity. You will never find an evil dog in literature. If there is one, it will likely be the tool of some higher power, or altered in some way, or portrayed more as a wolf than a dog, which is considered evil because of the time humanity spent as hunter-gatherers. Hence, the dog's role is two-fold; one, as a companion, and two, as godlike representation of childlike innocence.
I could expound on this, but anon probably doesn't care anyway.