The general observation from most long-term clinical studies of obesity, as far as I understand, is that many types of diets work to reduce weight in the short term, but the weight almost invariably comes back after the intervention period ends (usually within 1 year or so after the initial weight loss). Conversely, people who are normal weight and are forced on a hyper-caloric diet for a limited time (say, a few weeks) will gain weight as expected, but will naturally lose it back over the next period without explicit effort.

This has generally been treated as a lifestyle issue, but another possibility gaining traction is that "obesity" is a disease in itself, one that tricks your organism is seeking to maintain excessive weight, by impacting your appetite, BMR, exercise habits etc; this will of course usually make people suffering from "obesity" (for lack of a better name) over-weight, but the weight can be controlled without curing the disease. Basically losing weight doesn't cure your "obesity", just like when you suffer of diabetes, controlling your diet keeps you healthy without curing your diabetes.

The possible causes of this "obesity" disease could well be diet related, and definitely refined sugars seem to be at least a major culprit.