Depends on the person.
Most overweight, obese, or morbidly obese people are emotional eaters. But this only matters if they are under a lot of stress.
Now, most overweight people eat a relatively normal/healthy diet but don't get enough exercise. They probably feel too busy, or haven't found an activity that they enjoy. If they do exercise, then they eat too much of the wrong kinds of foods.
Obese people are like this, but it's probably both factors. They may have a terrible diet, or may never exercise.
Morbidly obese people generally have terrible diets, or have a poor sense of portion control. I sometimes wonder if it's a problem with their serotonin levels or some other mental condition. I think unhealthy eating can be an addiction, and deserves similar treatment.
Personally, I'm about 10-15 pounds overweight because I was raised on a carb-heavy diet and found PE/sports to be humiliating (I have a large chest and I'm asthmatic). My whole family was like that. I've recently become vegan and found exercises that appeal to me. I never knew that I was eating too much dairy and never thought about all of the bad carbs that I ate, and I never knew that pilates would make a big difference or that cardio didn't necessarily mean running. So I guess I'm fat because I didn't know that I had a choice.