RE: Why I think people are therians (opinion)
There really is no need to look for a specific reason why. Personally, I don't believe in a deity shaping our identities, nor in past lives, sorry. These questions of why, what for and how tend to the philosophical in my eyes. One should rather look for answers in our genetic makeup. Who we are is determined before birth (f.ex. who our parents were, where they came from) and after birth (f.ex. how we were raised, environmental triggers). Of course, what matters too, for the sake of completeness, is our basic personality.
While it's okay to ask yourself these questions every once in a while, you should note that the kind of answers you'll receive in the community are often, not always, looked for in the wrong areas of life.
Take a snow leopard for instance in the Himalayan mountains. Just assume you are this creature, that someone put you there in order to survive the upper colds and freezing temperatures. Wouldn't you rather be born in the desert, or on more fertile grounds? You couldn't, even if you tried. Whose fault is that? Parents? Nature/nurture? No, I doubt that is the case. The answer, my friend, is luck. If nature was a recipient creature and we could ask it why it put us there, it would probably say that we were born that way because of a strange current of events.
In my eyes, everything living and breathing on this earth is the result of that. People, wolves, sneps, they're all bound by a natural instinct for reproduction, a drive as deep as the four primary emotions. Why aren't some of us real live sneps? Because people gave birth to them... Turn it around the way you want or read it the way you like, but I'm afraid we were born therians because of (un)lucky selection.