I haven't become obese or excessively fat but I've gained a good 28 pounds my first semester of college. Drinking, eating more or less when you want with nobody family to tell you otherwise - things can spin out of control. I do look at myself and get sadness in the, "Man six months ago I looked like _____."
But speaking as the third party my older brother was morbidly obese from about 10-15. A hopeful TL;DR of it: A girl he liked rejected him and he just turned into a freak of nature (not in a bad way). He wouldn't eat with us at dinner if things were even remotely unhealthy, he wouldn't touch a slice of pizza, a potato chip, a gram of sugar - nothing. He would be doing crunches and leg raisers before going to bed (I know crunches don't meaningfully contribute to weight less, just indicating losing weight was on his mind 24/7). He would jog around our house and up and down the stairs during commercials for football games and TV shows, he would do push ups in between taking turns playing Madden and Tekken. Eventually he just... Got skinny one day. It's so weird I was never used to seeing my brother be anything but obese I never noticed him just get thin all of a sudden. Then he got into college and got absolutely jacked and he's been that way since. He can afford to have cheat days now but he's still kept his food very healthy.
I guess what I'm saying is yeah you're hitting the nail on the head. He didn't tell any of us he wanted to lose weight or to be considerate for him and try to eat better to make it easier for him. He just did his thing quietly and he just became skinny and caught us all off guard. If you're a bigger guy/girl I wish you luck in your journey.