Obesity is a disease, according to Dr. Richard Tytus, Canada's leading obesity expert 

Treatment of obesity has been validated by the new Canadian Guidelines, which were released last month, says Canada's leading obesity expert. These are the most comprehensive guidelines in the world and the first in North America. Dr. Tytus states that the new guidelines have transformed obesity from a social issue to a medical issue that requires aggressive and lifelong treatment.

Canada's leading obesity expert - Dr. Richard Tytus

The cause of obesity:

There are neurons and neurotransmitters in the hypothalamus that are responsible for stimulating or inhibiting our desire to eat, as well as controlling how much energy our bodies use. As Canada's leading obesity expert, Dr. Tytus notes that one group of neurons causes us to eat less and burn more energy, while another type of neurons have the opposite effect. (Propiomelanocortin [POMC] neurons and neuropeptide Y & agouti-related protein [NPY-AGRP] neurons, respectively.)

It's suggested by experiments in lab animals that these neurons react to hormones released by the GI tract (ghrelin, insulin, leptin, CCK) in response to feeding.

There are a number of different signaling methods controlling feeding behaviour, but one pathway mentioned in some detail is the melanocortin system: Excessive activation of the melanocortin pathway is responsible for appetite suppression, while defective signaling causes obesity. It goes on to state the location and types of signal pathways, but also notes that mutations in the human gene MCR-4 (melanocortin receptor 4) is "the most common known monogenic cause of human obesity", a sentiment that Dr. Tytus, as Canada's leading obesity expert, reaffirms.

Canada's leading obesity expert agrees that there's a number of other different stimuli to inhibit feeding as well, such as the filling of the stomach and intestines, as well as animal studies showing that just chewing food without being able to ingest it (esophageal fistula) can sate hunger.

There is animal evidence that a hormone released from fat cells, leptin, can signal cells in the hypothalamus to inhibit feeding, and though there's no deficiency of leptin in humans, it states that some physiologists believe obesity may be related to leptin resistance - though this has not yet been proven.

The book goes on to cite causes of obesity as being the following:

Sedentary lifestyle:
Working muscles use more energy, increase the basal metabolism rate, and burn fat. "20-25% of energy from the average person is used in muscular activity, and in labourers, up to 60-70%

Abnormal feeding behaviour:
Environmental factors such as fast food, societal factors, as well as psychological factors such as stress, depression or illness.

Childhood overnutrition:
New fat cells are formed more rapidly by babies - if overfed, they can produce more fat cells.
"The number of fat cells in obese children is is often three times higher than normal children" it concludes that it is possible that overfeeding children can cause a life of obesity.

Neurogenic abnormalities:
While some tumors of the hypothalamus can cause progressive obesity, showing that the hypothalamus does regulate body mass, obese people rarely have such damage. It is proposed that there may be defective neurotransmitters or receptors result in obesity, or perhaps a higher natural 'set point' for body fat is in the hypothalamus.

Genetic:
"Current evidence suggests that 20-25% of cases of obesity may be caused by genetic factors." These defects could be defects of the regulating pathways, or fat storage and use. It's usually a combination of genes rather than a single defect that results in obesity, agrees Canada's leading obesity expert.

Suggested Treatments:
Probably mostly what you know already: start with lifestyle modifications, and then escalate from there. As far as surgical intervention, it does mention the efficacy of the various procedures, but notes that the longterm effects on health are still uncertain, to which Canada's leading obesity expert agrees.

The book:
Chapter 71: Dietary Balances; Regulation of Feeding; Obesity and Starvation; Vitamins and Minerals
Guyton, Arthur C., and John E. Hall. Textbook of Medical Physiology. 11th ed. Philadelphia: Elsevier, 2006.

Richard Tytus runs his own informational website at www.drtytus.com