Re: Make it mandatory
The entire diet and brain thing is somewhat complex. There are multiple factors involved, some might be alleviated by teaching people bodily awareness, how hipstery that might sound.
Eating slightly too much over time can make you less susceptible to listen to the signals that tell you you are full. You get used to eating when you are full or eat a full portion when you are very close to being it. If your main meal was largely a certain types of carbs and/ or sugars, you also are more likely to feel "hunger" and cravings within a shorter amount of time than if you eat more proteins and/ or fat.
Sometimes the habit of overeating sneaks in slowly. You sometimes find youngsters who can "eat whatever they like" as they say and never gain weight. At that point their sense of hunger and fullness is appropriate to their activity level. But I think in some cases, they will little by little overeat slightly, because they don't need to think about what they eat. So they eat a chocolate bar here, more dessert there, slightly larger dinner portions and so on. Slowly they gradually become used to eating slightly more, and as they age additional weight will grow alongside them. Of course, there might be some metabolic differences between ages, but I don't think that's the only reason for weight gain.
There's also culture at play. Some have a very pushy eat more dinner, have another slice of cake environment. Abstaining from eating desserts like cake and cookies will often be seen as weirdo behavior, in worst case an insult. If such visits are frequent, if children grow up with it, I can see it as a contributing factor that will lay the foundation for a stomach that starves for more, even if misplaced so.
There's also the thing where your sense of taste is somewhat altered depending on what you eat. If you eat a lot of fats with sugary tasty foods and candy, cooked vegetables can seem quite tasteless in comparison and lose prioritization. Sometimes one gain the habit of always having a dressing, ketcup or other potentially calorie dense choices. Add a bit here and there and together with the food you eat in general you get enough plus in calories to gain weight.
I also don't think people notice or they forget how bad they feel on a bad diet. Loss of energy, mood, concentration. Occasionally their digestion will be a bit off, being so often bloated they don't notice it anymore, not ideal fecal patterns (ok I wont go there). If the bloating is so bad the stomach has difficulty digesting what has been eating, I did read once that the part of the stomach that is not getting (enough) nutrients might still send hunger signals/ cravings which people will be inclined to listen to and eat often more which will reinforce the problem. I can't tell whether that's true however, but there is some signaling going awry for sure.
And there might be some hormone thingies influencing, but I don't know anything about that.
There's also the fact that some people just don't care. They'd rather have their 5 minute of instant gratification, rather than a healthy and well feeling body on a regular basis. I know people who have noticeable stomach problems and still continue to eat a lot of crappy food.
There's also instances of food glorification and pride. Where eating bacon or a tripplemaxxx5000 burger is The Shit and it's really Awesome to eat which means you must be very awesome for eating it too. I remember my brother, thin as a needle, was always awed and followed by encouraging humor by the rest of the family for placing such large potions on his plate. Jokes about buying a bigger plate just for him etc. He's 30 and chubby now.
If you do try to change your diet and restrict, for instance, sugar in almost its entirety because you have great difficulty with controlling yourself if eating only some, some people will easily see you as hysteric or overly nazi when they see you thanking them no for their desserts and cookies. This also ties together with people being a bit pushy in telling people to eat more, even if that is meant politely and to disarm modesty in eating. I do think it's a bit of a bothersome tradition in any respect.
This reminds me a conversation I overheard between my mother and aunt just recently. We were going to visit my brother for coffee and they were discussing whether to bring over some leftover cakes. Because they didn't know if he had anything prepared and they'd have to have "something, if only a few pieces of chocolate". Having a visit with coffee and nothing else was too little to stomach. I was actually kind of surprised visiting relatives, I haven't really noticed before that I'm the only one above 20 who's not chubby or overweight on my mother's side.
And there's comfort eating. Which mechanics I don't have entirely mapped out, but it's there. There's also certain illnesses that make weight difficult to control, but those are more limited.
That's all I can think of at the top of my head, but there are certain to be more contributing factors, making a mess of it all.
And ofc, you can become overweight on healthy foods as well. It's easier to make examples of unhealthy foods, because their consequences are more easily recognized.
Edity: Oh right, I forgot to mention; There seem to be increasing evidence that shows the gut microbes have an impact on behavior and mood. So if your gut bacteria is messing up, you can become more prone to anxiety and depression, for instance. Feeling glum can further impact how much you care about what you eat and your choices of food, making it a reinforcing loop.
http://onlinelibrary.wiley.com/doi/10.1111/nmo.12198/full
