Fetishism is probably one of the most complex aspects of human psychology. Pretty much anything that creates a sexual or emotional connection to food or fat can lead to developing a fat-related fetish, from being scolded as a child for overeating to simply having had a fat sexual partner.
In my case, I think it started with a childhood attraction to pregnant women, which evolved to include fat women who looked pregnant. Sometime in my teens fat supplanted pregnancy as my primary fetish and grew to include many aspects of fat and weight gain, both in women and men/myself.
Good grief I wisssh I knew! Like I think mine came from watching cartoons of characters shapeshifting... I remember stuffing clothes when I was little pretending to be a bumble bee. I was taken by transformation. Dont know how I got stuck on weight gain, and before I knew of this fetish, I was consumed by weight loss and an ED. The years have passed, now at 28, Im finally going to gain about 50 pounds, not a large gain, maybe 60 idk, I normally set at about 125 pounds, so maybe around 200, I want to be curvy and soft. Ultra feminine. I just cant stand the curiosity any longer. Ive been really thin, but never thin enough. The next logical thing for me to do woukd be to gain... Ive been on a long journey trying to figure this out...
When a child watches countless cartoons like Pig's is Pigs, associating food, expansion, with euphoria. Then you combine seeing a pregnant women walking around cradling her big belly's like a pride full balloon with the look of euphoria on their face. Then you have birthday party's with helium filled balloons floating in a euphoric state. I have no doubt where my fetish all started.
I think the reason gaining appeals to me, is that as I grow fatter, and society deems me less attractive, my partner therefore becomes by far the more attractive one of us, and in my twisted mind, she gains all the power.