Obesity is caused by 3 simple factors:
Unhealthy diet (high-fat diet)
The first main cause of child obesity is an unhealthy diet (too rich in sugar and fats and not enough fibre and carbohydrate). These are usually the kinds that people buy from fast food shops. They include buttery foods, cheese, and lots of oils and salt. Some parents also cook foods like cake, bacon burgers, potato fries, ice-creams, and double cheese pizzas. Sometimes eating too much can also make you fat even if it's healthy food.
Lack of physical activity and exercise
Child obesity can also result from sedentary lifestyles and lack of exercise. This includes sitting at one place all the time, and not making any attempt to move around or exercise. e.g. sitting around a lot, or watching TV for long hours of the day. If this habit is not matched with regular exercise to burn off the fat, it can result in obesity over time.
Medical condition or medications
In many other cases, it could be a medical problem of not being able to control hunger or the effect of some medicines that you take.
If this is what seems to be a problem for you or a friend, it is most important to speak to your doctor immediately. Your doctor can always advise on what you can do.