Just like the the personal reason behind why someone is a vampire, I believe that there can be MANY many reason for someone to be an "animal-person", even some of the traditional ideas behind lycanthropes could fit in to a reason for why someone is a therian today.
[ Side note: Yes, I'm aware that there is a term called "clinical lycanthropy", but the word "lycanthrope/y" has many definitions, just like therianthropy, and zoanthropy. For this post, I'll be using the term to define wolf, and wolf-like animal shifters (with or without physical shifting traits). ]
What are some of your thoughts to why you believe you're a therian/shifter/lycanthrope/walker/whatever you want to call it.
Recently, I've found out that apparently I'm related to what is probably the Irish version of a werewolf. Two of my family names on the Irish half of me origin in the word "wolf" in Gaelic, and there are stories of wolf-people, wolf family totems, and people turning into wolves (both supposedly physically and astral/bi-local) in the same area of the country where my family is from.
My current idea is that maybe certain individuals have a genetic predisposition to be animal-people, especially those who are shifters. Another line of thinking to this is that maybe animal-spirits are attracted to those family lines where this sort of "practice", even if it's subconscious, is common. So basically, it's more convenient to choose someone who has the prior genetic "exposure", then to teach someone who has none how to work with it in this way.
Just something I've been thinking about.
Of course, this does not explain everything away. Many people hold the strong belief that it cannot be genetic or hereditary, so obviously keep an open mind about the whole thing. Not one theory is correct, because everyone is different.