Obesity is not "mainly" caused by some unpreventable condition or psychological problem. It is mainly caused by a lazy culture that makes it easy for people to eat badly, and for younger people, growing up being raised by people who fed them crap.
I don't like cooking. It would be easy for me to fall into the "lazy eating" trap if it weren't for the fact that I grew up on good food, and I feel ill eating crap, whereas most Americans today seem to think feeling that way is normal. So I have no microwave, and I have an intentionally small freezer, and I just generally make it really difficult for myself to rely on crap, and therefore I eat reasonably well. I like good food. I just don't like making it. So I make it nearly impossible for myself to make bad food, thus I eat better.
Personally, I have a demonic craving for sugar. This is very common in humans -- we need more sugar than a lot of critters do because of the energy demands of our brains, and it's hard to find in nature, so our drive for it is strong. I like to attribute my love of sugar to having an especially large brain. :mrgreen:
I have two ways I can address that. I can eat a lot of pastries and candy, or I can eat a lot of fruit. They both do the same thing. They both satisfy that craving.
I admit I'm weak for a good donut, but if I keep my place stocked like a monkey's horde, I will eat fruit, because it's there. So that's what I do. And some dark chocolate. It's good for you.
If I want my cookies, I'm gonna have to make them myself, and because I'm lazy, that doesn't happen terribly often. It's easier to eat the fruit. Even if the cookies do happen, at least I know they're made of actual food, not weird fillers and fake blueberries made to taste reminiscent of food.
People started eating crap because it's easy. Candy can be bought in great quantity, because it usually doesn't go bad. Greasy highly processed foods take less prep and are more instantly gratifying -- just throw it in the microwave. Corn syrup makes things cheap. They are getting worse because now they're born into families where people have always eaten that way.
Even people with medical problems are not simply relegated to ballooning out by 200 pounds, as someone else mentioned up-thread. I know a lot of people with such health issues. Over a dozen. An entire generation of my family got hit with metabolic problems and failures due to long-term toxin exposure, and I have several friends with various issues that can cause weight gain. None of them have gained more than 20 pounds. Most of them take their meds and they eat sensibly and many of them work out, and they have maintained a reasonable weight, with some staying within the normal range. Some have inched into the overweight category and can't get down lower than that, but being slightly overweight isn't a big deal as long as their lifestyle is good. The ones who don't do any of those things were already obese long before they had health problems.
Metabolic issues are not a cause to simply throw up your hands and let your bum spread. We have treatments for these things, and most people can control their weight to an extent if they put in the effort.