The balance in between the calorie intake & energy expenditure will determines a person's weight. If a person consumes more amount of calories than what he or she burns (metabolizes), the person will gains body weight (the body will store excess energy as fat). If a person eats lesser calories than he or she metabolizes, then he or she can lose their weight. Hence the most common causes of the obesity are physical inactivity & overeating. Mainly the body weight is the result of genetics, environment, metabolism, behavior & culture.

• Genetics: A person was more likely to develop the obesity problem, if one or both of their parents are obese. Genetics will also affect the hormonal levels which involved in fat regulation.
• Overeating: Overeating will leads to the weight gain, particularly if consuming diet is having high amount of fat. Foods which are high in fat or sugar will have high energy density.
• A diet high in simple carbohydrates: Carbohydrates will increase the blood glucose levels, which in turn initiate the insulin release by pancreas, & this insulin will promotes the growth of the fat tissue & can cause the weight gain.
• Frequency of eating: The relationship between the frequency of eating & weight is somewhat controversial. People who are eating small meals 4 or 5 times daily, will have lower cholesterol levels & lower and/or more stable blood sugar levels when compared with the people who eat less frequently (2 or 3 large meals daily). One possible explanation was that small and frequent meals will produce stable insulin levels, whereas large meals will cause large spikes of insulin after each meal.
• Physical inactivity: Sedentary people will burn lesser calories when compared with the people who are active. Physical inactivity was very strongly correlated with the weight gain in both of the sexes.
• Medications: Medications related with weight gain which includes certain medicines like antidepressants, anticonvulsants & valproate, some other diabetes medications , certain hormones like oral contraceptives & most corticosteroids like prednisone. Weight gain can also be seen in persons using some high blood pressure medications & antihistamines. The reason for weight gain with medications will be differs for each type of medication.
• Psychological factors:For some of the people, emotions will influence eating habits. Many people will eat extremely in response to the emotions like stress, boredom, sadness or anger. While most of the overweight people will have no more psychological disturbances when compared with normal weight people, 30% of people who are seeking for the treatment related to some serious weight problems will have difficulties with binge eating.
• Diseases: Like polycystic ovary syndrome, hypothyroidism, insulin resistance & Cushing's syndrome will also contributors to obesity.