Causes of obesity. Obesity is generally caused by eating too much and moving too little. If you consume high amounts of energy from your diet, particularly from fat and sugars, but do not burn off the energy through exercise and physical activity, much of the surplus energy is then stored by the body as fat.auses of obesity. Obesity is generally caused by eating too much and moving too little. If you consume high amounts of energy from your diet, particularly from fat and sugars, but do not burn off the energy through exercise and physical activity, much of the surplus energy is then stored by the body as fat.