I do enjoy reading people's villain origin stories ;-)
Here's mine, copy-pasted from another similar thread and lightly edited:
I was a fat kid and a "healthy eater". Like, my earliest memories are feeling nice and full and stuffing pillows under my top to make my tummy stick out even more.
Once everyone stopped finding me cute, I got put on a diet... with limited success as I'd still spend holidays at my grandparents, and they let me eat whatever, and let me sneak sweets on the side even though they said they shouldn't and my mother wouldn't like it. Anyway, I usually came home with a bigger tummy than when I left, and then got put on a strict diet.
But one year I must have gained more than usual or maybe just seeing me stick out my belly unexpectedly did it - anyway, my mother had come to pick me up, and I'd just run up to use the bathroom after lunch before we drove home...
I've always loved feeling full and I had a habit of petting my big round tummy. although I was self-conscious enough at that point to only do it when no one was looking. So like I said, I came out the bathroom upstairs, thinking everyone was downstairs. I was feeling nice and full and round so I had my shirt tucked up under my chin still and was sticking out my naked tum and rubbing it with both hands and admiring how round and soft it felt...
...when i heard my mother cry: "Heavens how on earth did you get such a big gut!" She'd come up to see what was taking me so long and I hadn't noticed...
Then she made me show my gran, lift up my shirt again and all, and said: "Would you just look at that big belly? Do you realize how FAT it is?"
She must have been completely shocked because she usually avoided calling me fat under any circumstances, she knew all the euphemisms in the book. I got a very long speech about how worried she was about me, and if there was a reason why I was eating so much on the ride home, and I was soooooooo embarrassed all I did was try and hide my tummy with my arms and suck it in, I could barely listen to her.
Well that did it. Ever since I've been both embarrassed but also insanely turned on by my fat tummy. Not even getting fat all over or gaining - really just feeling how big and round my belly is, and eating - overeating - to make it even bigger and rounder.
Like, nothing triggers the urge to gorge as much as feeling a bit bloated or noticing my tummy is a bit bigger than usual. And even when I see another fatty with a nice sexy belly, I want to feel mine, rub it, poke it, and fill it up till it hurts.
4 months