At least for eating, most people don't operate rationally; can you tell me how many calories you ate and burned today? yesterday? I can, but I have been putting a lot of effort into this for some time now. most people don't know what they ate yesterday, unless they eat the same thing every day. Most people operate on instinct. I've operated on instinct for most of my life, too; I was super skinny as a teenager, healthy in my early 20s, then growing into overweight but not obese in my late 20s and early 30s... then obesity set in during the mid to late '30s
This really was pretty steady; my strength changed a lot; during that period my bench press would go from 50kg to 100Kg and back again multiple times, but my weight just continued to very slowly increase.
Clearly, whatever subsystem decides "I'm hungry" or "I'm full" is tuned to get me to eat just a little bit more than I need to maintain my body. And it's pretty good at it. If I work out? the more I work out, the more I want to eat. working out without caloric control simply doesn't work to lose weight for me.
I've been counting calories in and out (and trying to run a small long term deficit without otherwise spending a lot of effort on what I eat) for some time now. I have gone from a BMI of 31 to a BMI of 26 in the last year or so. This after trying a bunch of "eat this not that" diets, which have never worked for me. But I have been... slightly hungry for the last year or so, and thus I think about food and my relationship to food a lot.
My theory? My theory is that my farming ancestors found it advantageous to eat a little more than they needed when food was around; that it was better to carry that bit of a belly around all the time if it meant that you wouldn't die if you didn't stash away quite enough grain for the winter.
(As an aside "just eat less" "stop when you are only slightly hungry" also doesn't work for me; that ends up with me eating way less than I need to for some reason (I need to come within 20 or 30% of my maintenance caloric intake, from experience, or after a few days I enter a semi-fasting state where I'm tired and can't focus - I'm guessing this is also the farming ancestors. "Oh, there isn't enough food stored? Maybe you should take a lot of naps until this year's crop is ready to bring in")