Eat less, exercise more. Simple? Yes. Easy? No. If weight loss were easy, obesity rates among adults in the United States probably wouldn't have reached the current 36 percent. (from Kasier Health News.)
That's the "official" view on what causes overweight and obesity.
Gary Taubes (and my physician, by the way) have another view: massive amounts of carbohydrates creates an insulin reaction, which causes the body to store fat, and to create more fat out of the carbs.
I know that the second view has helped me lose the weight, and I am still losing.