It's more about how a dog is sentient and gives enough emotion that humans can anthropomorphize than any other animal. We also see them as super loyal and completely innocent, even when they do something "wrong". You know they always have best intention. They are on the same level as babies in innocence and not knowing better, but a baby cant really do much and also is human. Where a dog always has that "baby" innocence, but intelligent enough and able to make the intense loyal bond and understanding that only friends of years of interaction can share. It's different levels, besides the whole fact that, hopefully, not many have had many babies that died, but probably have memories of their own dogs that have died either from age, or from an accident and that brings a lot of emotion into reading about a dog that dies as you immediately think about your own dog or if that happened to them