embsmu:
I think we're more or less born with our fetishes. As to why, maybe to encourage genetic diversity?
Psyman:
Personally, I didn't get into fat appreciation until later in life. I was always overweight and very self-conscious about it. I knew I loved to eat and that I didn't like to diet and exercise, but I didn't like my body nor did I believe that anyone was capable of finding me attractive.
This says it pretty much for me, except that I did like physical activity: weight lifting, running, biking. I did it because I like it, though I'm limited now, not by my weight, but by back surgery.
It was really only in the past few years that I let myself gain weight and like how I look being fat. I've always been a fan of bhm. I realized there are too many good looking bhm for there to be anything wrong with it or be ashamed of.
As much as I used to dislike being overweight, ironically, I'm having a hard time gaining. As for why I like it, I have no idea. All I know is that fat guys turn me on, and I get aroused identifying with them.