because eating is vital to body, so the body has defences built in to keep the supply going at almost all costs. metabolism to some extent is and works through the human nervous system. This means you defences are mental and physical, that and about a quarter of what is eaten is for the use of the brain. it requires a tremendous amount of energy to operate and function.
Our bodies are programmed to stop them from working.
It's not because it 'thinks' you are on a famine, if you know you are on a diet, and your defences operate in the brain, why do you think your body wouldn't know you are on a diet?
Why would that information start in the mind and stop in the body? How do we even know the desire to diet is always wholly mental? I'm sure your body can tell the difference between being fed and not being fed, enough for you to have to dodge hunger.
Our views on slimming are confused because of the assumption that dieting is right and good, when it's actually disorder. You need to suspend that and assume that it is possible that the body fights dieting for a direct reason, not as a mistake.
Why would it fight apart from this? To prevent anorexia, what is the difference between someone who develops anorexia, and one who does not? One of them succumbs to dieting, the person with anorexia and the other doesn't, mainly.
A lot of people don't think about how much pain keeps anorexia at bay, how much more natural and prevalent it would be otherwise. We assume we'd just stop when enough weight was lost. But those who have been on the cusp or earlier of a serious eating disorder know that as you lose weight, your perspective can alter tremendously and what seemed slim from where you started from becomes not so slim, or at all, when you get there or there abouts.
So dieting failure us about our design, not the way we think our sense of free will. It's to do with the way we work and function. Dieting doesn't work and isn't going to because of all this.