I think it's definitely multifaceted. In my case, yes, I do eat too much, and yes, I am morbidly obese, and yes, my insulin resistance is much improved by weight loss.
But WHY does a person eat too much? That is the question: why?
Anyone who has experienced as much across-the-board vicious hostility about her weight as I have -- including everything from threats of violence to actual attempts to run me down in a car not once, but twice, by total strangers screaming anti-fat epithets out their windows as they near-missed me in a marked crosswalk -- would have to be insane to eat so much and incur so much abuse for it, but the drive to eat is like the drive to breathe, the drive to seek water when thirsty or the drive to avoid pain. It's a very, very, very strong biological drive.
Why would a human being -- or any other animal for that matter -- feel such a powerful biological drive to eat more, more, more when any objective observer can easily see and measure that:
1) there is already plenty of bodyfat on-board to use for fuel, and
2) a large quantity of total calories has already been consumed.
It's complex and I have no doubt that genetics, toxins in the environment, stress hormones, reactions to substances in the diet, cultural pressures and habits, etc. all play a role.
Cultural norms are a big part of the obesity-driven T2 epidemic, e.g. no one is walking around at work offering everyone a syringe full of heroin, but it's perfectly normal behavior to walk up to carb-sensitive people at work and try to persuade them to have a cupcake or some fudge, "C'mon. Just have one. I made them special. You can have a few bites. Take one!" No one is allowed to put booze in the punch bowl at afternoon work parties anymore, but mandatory lunch-time meetings serving only pizza, chips, soda and cookies is perfectly normal, acceptable work-place behavior.
As I've improved my diet and made drastic changes to how I eat vs. how most Americans eat, I've had to do things like sit through an entire impromptu social gathering (several hours) with nothing to eat or drink except water while the people around me gorge on simple carbs, processed crap and booze -- not a clean vegetable or protein source in sight.
When your university-educated aunt thinks that green Jello counts as a vegetable, you realize exactly how sick our national food culture has become.