It's interesting to note just how different everyone's experiences are with their feelings around "getting fat." Like the OP, I was pretty slender most of my youth. It really wasn't until my late twenties that extra weight started piling on. In my thirties when I got my first real "cubicle job" I suddenly became much more sedentary, spending most of my day sitting at a computer, and eating lots of take out food.
While I always had a love for fat and was curious about gaining, my gains were not intentional at first. The weight really did "creep up" on me, and I noticed the little changes first. Feeling a bit softer in the belly, noticing my clothes fitting tighter, etc. Then at some point, I looked in the mirror and saw how round my belly looked, the way my polo shirts just kind of draped over it and looked like a big tent.
Then it became undeniable. "Wow, I think I'm officially fat now."
I remember the feeling of pride and acceptance I felt when someone else actually called me "fat." I met this group of big guys when I was on a work trip, and we were hanging out at a bar when one of them said something like, "Hey, us fat guys gotta stick together."
I like that.
It brought me a lot of pleasure to lie in bed, feeling the weight and the softness of my belly, rubbing it and patting it. I feel sorry for those who aren't able to find so much enjoyment in their own fatness. To me, it has been very comforting and satisfying to get so big and soft, and even though I've had to slim down a bit because of health issues, I am very grateful to have gotten as heavy as I did, and to still have a little bit of a belly.