Some really good discussion above - I think this shows what a complex topic obesity is, touching on everything from physical health, to mental health, incentive, etc.
One thing that shouldn't be overlooked, in my opinion, is the "moral hazard" of obesity. In particular in Canada, one does not pay for the costs of their poor choices, making it easier to make those poor choices. This exists with smoking and related health costs, and exists with obesity, as well. The cost to our healthcare system of obesity is tremendous, and borne entirely by the public coffers, not the individual. Not that different outcomes have been observed, for example, in the U.S. where an individual may bear some of those costs, but I do think this plays a role. Unlike a lack of education, where you bear the cost in terms of lower potential earnings, as a different contrasting example.
TrickyRicky also makes a very valid point about mental health, which has largely coloured how we speak and think about obesity. My mother is obese, and it's directly tied to her anxiety and depression. Food is a form of self medication for many, and in those cases is as much a disease as drug, nicotine, or alcohol addiction.
On the topic of CICO (Calories In, Calories Out) which includes both sides of the equation, this is the single proven and sustainable "diet / lifestyle" to maintaining a healthy weight, yet we don't teach it well. I would guess less than half the population has ever considered what their own Total Daily Energy Expenditure (TDEE) is, let alone considered what their average caloric intake is. Many of us think in terms of "eating clean"and know inherently when we are eating too much over a period of time, but without explicitly understanding this in terms of caloric intake vs. expenditure, continue to spin wheels in an effort to balance this equation.
Just some thoughts, on what is a very difficult topic.