I know it sounds like common sense, but I think a lot of us grew up with parents that made us finish our food even if we weren’t hungry. So eating more than we needed was normalized, and it became difficult to just stop mid meal.
Growing up with not much also conditioned you to eat everything on the off chance you can get a full meal. This has been a point of contention with my SO several times. They came from a well off family and they have no problem not finishing their food or not using some ingredients in the fridge whereas I've been conditioned to finish all meals no matter what and to be able to use up everything (not letting stuff expire) stored in the fridge.
Strong "you can't let good food go to waste" in the post-war generation, including in my own family. It's so ingrained even in the next generations that many of us will just "finish their plate" even though there's no necessity there. Some of us are quite well off now, but attitudes around food haven't changed. You have to finish your plate. You can't let good food go to waste. People elsewhere are starving. People worked hard so you could have this food. You don't know when you'll be able to have a nice meal like this again.
Like you, I realized the difference when I met people from different, well off, culturally food-secure backgrounds. They'd just stop eating, and throw the uneaten leftovers in the trash. Doesn't matter how good the food was. Doesn't matter how expensive the food was. Doesn't matter that you could eat the leftovers later.
I had a really hard time landing on some reasonable middle ground (you can save leftovers, but you're allowed to stop eating when you're full, etc.). Made me realize that it's so much more cultural than personal. Also raises questions about what we're going to pass down to the next generations, intentionally or not.
It's also not rocket science to figure out, how much you need to eat and plan your groceries accordingly. I seldom throw away food or let it go to waste and I really dislike, if others do.
Same. I also don't get the people saying it's because they're conditioned to clean their plate. Why are you putting so much on your plate in the first place? I get if you're 7 and your mom's doing it but an adult should be able to manage their portion sizes.
The great depression shit again? Still is a factor in this? It cannot happen again, it factually cannot. We are more efficient at growing food than we've ever been.
Definitely could happen again, and will eventually happen again. All civilizations fall eventually. Current food production, transportation, and preservation is almost entirely ran on fossil fuels (fertilizer, pesticides, machinery, refrigeration, etc). Climate change will eventually make a significant proportion of current productive land unproductive. Climate change, topsoil loss, super-bugs, super-weeds, etc are also already causing problems; and many experts think the current way of doing agriculture are unsustainable.