OK, here is what I have been told, based on the experience of wildlife biologists and such. In a nutshell, the soft organs taste best/have the strongest flavor. It is why the internal organs are consumed first after the usual gut evisceration during a kill. Now, first of all pets are not wild animals, and also usually the internal organs are not exposed when the human dies in such a situation. However, the eyes and tongue are easily accessible and tasty, so they go for that first. After that they simply consume the surrounding tissues. Also, they tend to be leaner areas, with less fat reserves. They love their people, but they also do not have the social prohibitions we do during survival situations. Now, I am not an expert in this so please do not quote me but from my understanding this is why they start there.