I like the analogy, because I can personally relate to depression. I don't know that it is a great one, though.
We know very well how to cause weight loss. It's a calorie deficit issue, and we could force it to occur. That is, we can will weight loss.
It has not been my experience that people can will happiness—not even for a short time. They can (sometimes) will themselves to be productive, and smile, and go to work, and even drudge through exercise. But willing happiness is not a possibility I am aware of.
It isn't my argument that we should "force" weight loss, only that we can. We should be as sciency as we can be in order to come up with more convenient and reasonable ways to help be lose weight. I gamified it. I used some LW-ish principles.
Questions:
I assume you do not consider depression a choice. That is, depressed people cannot chose to become undepressed. They may choose to engage in behaviors that alleviate depression, but certain people are so severely depressed that they cannot summon the will to even engage in the depression-alleviate behaviors. Is this an accurate summary?
If someone's caloric balance were 100% controlled so that they had a 300 kcal daily deficit, what would happen to that persons weight over the course of 30 days? 90 days? 1 year? What would happen to their appetite? Metabolism? BMI? Assume they are given a careful balance adequate nutrients. Assume they are given freedom to exercise and be active to their heart's content. An exact 300 kcal deficit is alwasy 100% enforced. What would result?
I think you've rescued the rule that depressive people can't just decide to feel happy. But by your theory, they should still be able to go to work, maintain all their relationships, and otherwise behave exactly like a non-depressed person in every way. In practice this seems very hard for depressed people and a lot of the burden of depression is effects from not being able to do this. The metaphor that just as this is a hard problem and worthy of scientific attention, so weight loss can be a hard problem and worthy of scientific attention still holds.
But ...
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.