I think the best answer all around was in the first response: "personal choice". But after reading about therians, I might be somewhat closer to that. I chose a Dragon to represent me (I don't actually identify with being a "furry" for certain reasons) simply because its my favorite animal and has caught my fascination in recent years.
The reason I'm into this fandom is because I find anthropomorphic creatures fascinating and challenging to draw - in a fun way. I may adopt several OC's (but I wouldn't call them multiple fursonas) depending on what I feel like drawing or creating.