I know Ezra is paraphrasing someone else's argument, but this is really good:
This reminds me of Charles Karelis's "The Persistence of Poverty." The basic argument is that the wealthy misunderstand the mental state of the poor, which leads them to make conceptual errors when creating policies to address poverty, or, in this case, obesity. Think of a bee sting, he advises. If you have a single bee sting, you'll go buy some salve to take away the pain. Now imagine three bee stings, a sprained ankle, a burn, a cut, a crick in your neck, a sore throat, and arthritis. Does the bee sting matter anymore?
Karelis argues that this is more the situation of someone in poverty. Obesity is bad, but it may be just one of many bad things. Overdue bills. A horrible part-time job. Endless commuting time on the bus. A mother with diabetes. A child running with the wrong crowd. A leaking roof. In that scenario, slowly reversing your weight gain might be a good idea, but it hardly makes a dent in the overall crumminess of the conditions. It won't replace pain with pleasure. So you do things that are surer to replace pain with pleasure, like have a delicious, filling, satisfying, salty, fatty meal. That may make your overall situation more unpleasant, but then, making that situation pleasant didn't seem like an option in the first place.
This, he would say, is fundamentally different than the situation of someone who is fundamentally happy with his life but thinks he should lose 30 pounds. For that person, those 30 pounds are the main thing standing between him and perceived happiness. It's one bee sting instead of a dozen ailments. The condition seems manageable, and so it gets managed. Conversely, if the aggregate condition does not seem manageable, people are less likely to manage any individual part, because it will not bring obvious reward -- life will still be pressuring and difficult. The things that will bring obvious reward, however, often make the underlying situation worse -- think spending, overeating and drinking. But then, that's why they call poverty a cycle, and obesity fits there, too.
I think that basically sums it up, and jives with my own experience. It's a lot easier to drop that 30 when everything else is going well, as opposed to when you're worred about the kid's school, your ability to make rent, and the fools on the corner.