Just a few reasons why some people are overweight:
1 ~ food intake does not match energy output. If you aren't burning the calories you take in they are stored at fat in the body.
2 ~ exercise (or lack of). When you exercise or stay active you build muscle and burn fat. Lack of exercise creates weaker muscles & more fat.
3 ~ slower metabolism (or burn rate of calories)
4 ~ diet and food choice. Some foods are empty calories that just beg to be stored in the fat cells of the body, while other foods help to burn fat cells due to them taking more energy to burn than their calorie count offers.
5. Genetics ~ some people are born with the predisposition to have an extra layer of insulation. That is not to say that they must stay this way, only that their body was programmed to store fat effectively and efficiently given their DNA make-up.
6. Health conditions ~ Thyroid issues, Graves Disease, Crones Disease, Diabetes, and a myriad of others lower the ability of the body to function at it's optimal fat burning level.
7. Medications ~ Many, many, many medications or prescriptions people take for illnesses, diseases, or conditions have a side effect of causing weight gain, lower energy levels, lower metabolism, and more; all playing a part in the patient's ability to maintain or keep from gaining weight.
8. Eating disorders and other mental health disorders or illnesses that cause a person to either over-eat or not care what they look like, or not realize what they look like. I had many mentally ill clients that did not recognize that they were over-weight. In their mind they were thin and looked good, which to them meant that they could eat however they wanted and choose not to exercise or follow doctor ordered plans to keep their weight issues under control.
Alright, well, I'll stop there... I think that's a good start. Hope this helps shed some light on the issue for you.