I got fat early in elementary school, much to my mom's chagrin. Even as a kid, I loved to eat and to be full, and I was fascinated when my belly got bigger. By middle school, I was obese, and I just kept getting fatter through high school.
I've always enjoyed food, and I love the feeling of a belly that's been pushed to its limits. That "couldn't contain one more bite" feeling is absolutely rapturous. While I haven't stuffed in a long time, if I could, I'd be packed to my limit all day, every day.
I also like the way I look as a big person. As I've reached extreme obesity, I've become more confident in my appearance.
Finally, there's a "challenge" aspect to gaining. At one point, I lost to my lowest adult weight. Obviously, I disliked it, because I've intentionally put on 450 pounds over the years since. In my regain, I've become verrrrrry round and carry a massive amount of visceral fat for someone my size. My sitting waistline is shockingly big, and I've enjoyed just how large my belly has gotten in proportion to the rest of my "also fat" body. If I could set some kind of world record without decimating my health, I swear I would.
1 week