Obesity could be caused due to simple overeating or due to certain physical disorders, psychological causes, or due to any of the several other factors that will be discussed in this article.

Certain physical disorders can result in obesity. For example, metabolic syndrome is a physical metabolic disorder that causes obesity, hypertension, high lipids and diabetes. Hyperthyroidism is a physical disorder that causes obesity when the thyroid gland is inactive. Other common physical disorders that result in obesity include: familial obesity, Cushing's disease, PCOS, edema, insulinoma, pseudohypoparathyroidism, Prader-Willi syndrome, Laurence-Moon-Biedl syndrome, hypothalamus tumor, and reactive hypoglycemia etc.

Some people turn obese because of psychological reasons, such as emotional eating. Yet others could gain weight and become obese under the influence of certain types of brain disorders that induce the peculiar habit of excessive eating. One example of such a brain disorder that causes the patient to overeat is encephalitis, which is a serious viral infection of the brain that causes brain inflammation. Other brain disorders that induce overeating include: brain injury, third ventricle tumor, certain brain tumors, chromophobe adenoma, and craniyopharigyoma etc.

Factors causing modern age obesity include: living a sedentary lifestyle, smoking for a period of time, living a stressful life, eating disorders, untimely eating habits, junk foods, being a couch potatoe, depression, and weight cycling, which is caused by several failed attempts to lose weight.

Other causes of obesity, weight gain, increased eating, etc. include: myxedema, mental retardation, lupus nephritis, klinefelter syndrome, idiopathic edema, hypogonadism, hyperandrogenism, heart failure, Hashimoto's thyroiditis, Frolich's syndrome, dilated cardiomyopathy, depressive disorders, chromosome 5q duplication syndrome, Bardet Biedl syndrome - type 2, Prednisolone, pituitary gland disease, thyroid hormone plasma membrane transport defect, Sohval-Soffer syndrome, polyneuropathy, pituitary tumors, MOMO syndrome, lymphomatus thyroiditis, HAIR-AN syndrome, congestive heart failure, adrenal cortex neoplasms, trophoblastic cancer, type 2 diabetes, pituitary cancer, Mauriac syndrome, masculinisation, hyperadrenalism, food addiction, and a host of several hundreds of other possible diseases or disorders or malfunctions, could be the cause of obesity or excess weight. 

Environmental factors that induce obesity include: lack of gym facilities, or parks or jogging tracks or even sidewalks, hectic work schedules, large portions of food, unavailability of healthy food, advertisments promoting unhealthy food etc.

Intake of certain types of medicines, also results in obesity as a side-effect. Corticosteroids, antidepressants, and seizure medicines are examples of medicines, which when taken promote rapid weight gain. The main reason why in-taking these medicines causes weight gain, is because they slow down the digestive system, reduce the body's metabolic rate, increase hunger pangs and promote water retention.

With increasing age, once you're cross the age of 25, the body begins to gain one pound of excess weight for every loss of one-third of a pound of muscle weight. Therefore, weight-training is important to reverse this process that occurs as a result of aging. 

Pregnancy is another reason, why women gain weight after conception, as women overeat for the proper development of the fetus, but find it hard to shed all that weight post conception.

Finally lack of proper sleep is another reason for obesity.