Sadly, energy expenditure seems to correlate inversely with what's required. When I was a kid I was happy to sit and watch cartoons all day, requiring almost zero energy. But now that I need it...
A massive amount of energy is just used by the developing brains, so you are still using that as you watch a cartoon.
"The growing brain is likely the key energy sucker in little kids, says biological anthropologist Chris Kuzawa of Northwestern University. Kuzawa did not participate in this study, but in 2014 his team found that the brains of young children consume a stunning 43% of all energy used by the body."[1]
The short answer is lift weights (resistance training) to increase muscle mass and consume caffeine. Both of those will increase your resting rate metabolism.
Well, some conditions could probably cause it, but those are conditions are rare.
Considering obesity is headed towards affecting 50% of the population in developed countries, those conditions are not a valid excuse for the vast majority of people.
For this vast majority of people, it's lack of exercise and mental state, as you say.
I turned 43 today. I probably exercise more than when I was 23 and I definitely eat much more carefully - no processed crap anymore, no alcohol, very little sugar. My insulin sensitivity seems to be almost perfect (at least the graphs produced by continuous glucometer say so).
I am still a bit worse off around my waist than I was twenty years ago. Not dramatically so, but at least 4 inches, perhaps 6. With lifestyle like that, I would have been absolutely lean in my early 20s. Something subtle must have changed within.
Dad bod is a real thing; I've heard explanations that relate it to the "parental stage" of your life. The body accumulates energy reserves for the sleepless nights & bouts of fatherly activities. Probably happens whether you have kids or not; a dad bod (to a degree) doesn't have to signal poor health. Just changing priorities.
FWIW, I haven't changed my weight or body proportions since I was 17, except a few cm in the waist. I turned 40 this June, two kids in.
I’m leaning more towards some chemical in our environment that is messing with our signaling hormones for hunger and satiety. There isn’t much evidence that people are getting less exercise than they were 50 years ago.
