[0] put about 30lb on during the pandemic period that I'm working off again.
I lost about 30kg (105 -> 75) by eating less, excersing more, all in about the span of a year. I never stopped feeling hungry overall for this period, constantly had to fight the urge to eat more. In periods where high stress reduced my willpower, I started putting back weight. I've put on about 10kg back in the following 5 years, so on balance I'm still in pretty good shape, but the trend is clear.
The mystery is why I have this relationship with food, and why other people don't. Most thin people are not going to bed dreaming of tomorrow's meal.
Unfortunately, loads of people in this very thread are disputing it. This submission is based upon the bizarre hope that there's a mysterious external influence that's actually causing people to gain weight, and not the merely coincidental simultaneous rise in caloric intake.
The psychological element is complex and profoundly difficult. That is without question.
I believe that the problem is not that different from the problem of alcoholism. Everyone lives in a society soaked with alcohol, some people do not feel an urge to drink at all, some people manage to keep their consumption in healthy limits for 50 years, some end up as hopeless wretches in 10 years.
The fact that you can force yourself to eat less than you feel the need to and lose weight in the process does not prove, as you seem to think, that weight gain is simply explained by people being weak willed and eating burgers instead of salad. You still have to ask why this change actually occurred, since it seems that it has occurred in the general US population about 50 years ago and most of the world has been following suit.
People in cushy office jobs eating as much calories as they want in 1900 are much less obese than similar people in 2000. How come? It's not that the 1900s office workers put in more effort or attention to limit their calorie consumption and burn more calories, it's entirely the other way around.
People in another culture eating a very sweet-heavy diet (and eating as much of it as they want) apparently consume much less calories than similar people in USA. How come?
The psychological element apparently wasn't as difficult some decades ago - people in 1960s who had as much food as they wanted, and the same kinds of tempting calorie-rich snacks available, were much less obese not because they were better at overcoming some psychological difficulties, but because apparently much fewer of them had such difficulties to overcome. How come?
It would be valid to reduce weight gain to "How to eat as much calories as you should instead of as much calories as you intuitively want" if and only if wanting to eat much more calories than you should is some innate, natural thing. The article points out that the spread of this tendency is a novel thing, it used to be rare, and perhaps it can be made rare again, so it's worth investigating the cause of that unhealthy appetite miscalibration instead of having people fight through the symptom (which they're failing at, because it's hard).
If someone's organism was working properly, they should not need to pay attention to CICO as the body will balance both "CI" and "CO" to get a decent non-obese result - the experience of earlier times and other cultures shows that the human body almost always (96% of non-obesity) does that naturally. If now the same mechanism is failing for 40% of the population instead of 4%, that is not caused by a change in ignoring CICO, people were ignoring CICO hundred years ago as well.
I eagerly await the day science figures out how to stop my food cravings and delivers it in a form I can obtain easily.