It's really a combination of a lot of things -- genes play a role, but lifestyle, eating habits, exercise habits and childhood obesity play a role
In Dp's case it is fairly normal that athletes like him face
My wife has a high metabolism so can eat twice as me and she is a 110 pound slim lady while I am 190 lbs 6'1" with a far slower metabolism
I need to eat less than her and I also need to eat less carbs than her
That being said, I can put on muscle far, far more easily than her brother (who has the same genes) - so there are pluses and minuses.
That’s similar to what the book presents. The head of the Eating Lab at the U of M, Dr. Traci Mann, began her career in psychology. There are many physiological and psychological factors involved.
When I struggled with weight (high school and college years) I was trying so hard to be thinner, but dieting backfires. Not just for me but for almost everyone. I wasn’t super obese, just heavier than I should have been. Diet companies count on repeat customers to make a profit. My mental health was probably a wreck as well.
My take away from the book was veggies first, move more and not everyone’s natural size is skinny. I have to be careful with my child who struggles. A part of my problem was my dad constantly hinting that I needed to lose weight. I would eat more just to spite him (hindsight reveals much).