Why do teenagers become obese?
1. Genetic factors

Your parents and grandparents also play a role, not in how they act but in the genes they have given you. If one parent is overweight, there is 50% chance the children will be obese. If both parents are overweight, there is 80% chance the children will be obese.
Metabolism, which affects how we store and use energy, is also controlled by heredity to a certain extent. Some people may also be obese specifically because of a genetic disorder. Therefore, body shape is not solely controlled by an individual.

2. Unhealthy diet

Frequent consumptions of high-sugar and high-fat food, such as snacks, fast food and much meat and little vegetable, will easily result in excessive calorie consumption, which will quickly be transferred into fats.

3. Sedentary lifestyle

Adolescents and teenagers spending longer hours sitting, watching TV, playing video games and other quiet pursuits but lack exercising. This combination of less exercise and more sedentary activity creates an unfavourable ratio of calorie consumption to energy use.

4. Stress

Obesity is related with mood. Competitiveness at school, relational complications with friends, family or dating partner create stress. Some adolescents develop overeating as an inappropriate resolution to manage stress.

5. Health problems

People with the diseases below will be more likely to be obese:
• hypothyroidism or diseases of the adrenal gland,
  growth hormone deficiency
• neurological diseases and mental handicaps
• abnormal skeletal development
• side effect of some medicines
• psychological and psychiatric disorders