It goes without saying that every person in this world has beast tendencies. We are animals regardless of whether we are Therian or not, a fact easily forgotten by many people who would have the label "animal" be distinctly inhuman. I think these people are rather shortsighted and lack wisdom, it takes a person accepting the whole truth of themselves to really know something in my opinion.
Part of that truth is that humans are animals. We are territorial, cranky, and prone to overstimulation just like anything else (some more then others!). Perhaps I cannot speak for everyone here, but I find that therian identified people are particularly and painfully aware of their own status as animals. Fundamentally we identify with things that aren't human because they are not bogged down by some societal standard of "civility". The dog likes to roll on the floor and get rubbed on the belly, so would anyone if only we didn't feel like we'd be judged for it. Like all bad things in life, this outward disgust towards "acting like an animal" comes from the deep, destructive insecurity that plagues human beings at large. Why not be a dog?
Of course, therianism is like all ideas, bound to a spectrum. It's expressed in different ways and understood differently as well, so who knows why or how it truly manifests? I think it's inherent in all people, is all. Its just a matter of accepting that fact.