Obesity must have a genetic component, but regardless of that, eating too much of the wrong thing can cause obesity whatever the genetic make up.
I see obese people in the mall drinking sugar-loaded soft drinks, and eating high carb junk. How can their obesity NOT be caused, or made worse by their diet?
I am inclined to be fat, and am currently size 16 or 18 depending on the brand. I was getting fatter since retirement: before that I was size 14, but now I have cut out carbs, especially food made from grain, such as bread, and also sugar, and I am slowly losing the excess. I eat as much meat, fish, eggs, saturated fat, butter, olive oil, dairy, and vegetables as I like. I eat fruit too, but not TOO much as it has too much sugar in it. I never go hungry.
I avoid processed food and anything that has to be made in a factory, hence butter not marge. I try to avoid chemicals and additives. Just keeping it old fashioned and natural seems to work. I'm 67 and in great health. Since eating like this for the last few months I have become more cheerful and less prone to depression too. My husband refuses to change his eating habits, and he is somewhat depressed, has insomnia, and often feels unhealthy. However, he has taken up one good habit of drinking lots of water, and this is bringing his weight down.
Some lucky people can eat all the fattening junk they like and still stay slim, but there aren't too many of them. Most of us pay for it if we indulge in the wrong stuff.