The prestigious journal "Nature" just posted a very interesting interview entitled "Obesity: Causes and control of excess body fat."
In a nutshell, molecular geneticist Jeffrey Friedman argues that obesity is mostly caused by our genes (70-80%), and to a lesser extent, our lifestyle. However, he explains that even a small reduction in weight has large health benefits.
As individuals, we all come in different sizes, and it is difficult to modify our natural size significantly to achieve an unrealistic ideal, as proved by yo-yo dieting. This does NOT mean that we should not eat heart-healthy foods and exercise. All it means is, do not try to look like the model in the magazine; learn to accept your natural body size that results from eating healthily and getting moderate physical exercise.
The article also points out several gene mutations that predispose individuals to morbid obesity. In other words, morbid obesity, like many other illnesses, has a genetic cause, and those affected should not be blamed for their condition. In most cases, their lifestyle is not the cause of their being severely overweight. Just like there are lucky people who can eat and eat and stay lean (and I know a few!), there are those who gain weight disproportionally to what they actually eat.