Genetics
I think a lot of the resistance to the idea that weight has a strong genetic influence is the fear of losing control. If you accept the idea that your size is genetic, you must accept the fact that you probably can’t change it–whereas if you persist in the myth that anyone can be thin if they just eat right and exercise, you can remain confident that your body size, unlike so many things in life, is under your control.
To me, the idea of realizing your lack of control is scary, but at the same time empowering. It takes a lot to accept the fact that no matter what you do you can’t change something–but at the same time it allows you to abandon exercises in futility and instead concentrate your effort in places where it will make a difference.