There are a number of homeostatic ("standing the same") processes in the body. One of the most critical is the one that maintains the proportions of sodium and potassium in the body. Every cell in the body depends on this proportion staying within certain tolerances. There are enchantingly clever mechanisms to keep it that way, and the body will undertake draconian emergency measures if it seems to be really going out of whack, cannibalizing bones, organs, and other tissues to get what it needs.
This process rises to our consciousness only in the veiled form of feeling thirsty (too much sodium) or craving salty food (not enough sodium) or possibly craving chocolate (not enough potassium). Most of the time we have no consciousness of it at all. It just works. Similarly, our breathing is regulated by the levels of oxygen in our blood. If there's not enough, we breathe more deeply. Too much, we breathe more shallowly. This process too just works. We don't make notes on our calendar: "must breathe more today." Or "not going for run: remember to cut breath intake this afternoon!"
There are actually dozens of such systems in the body, regulating all sorts of things. One thing is conspicuously absent in all of them, and that is participation by the cerebral cortex. All of them "happen automatically." You don't have to think about maintaining the Ph levels in your body. If they shifted minutely, you'd be quite dead. But the parts of the brain that do this monitoring, being very primitive, and essentially the same in iguanas as in us, don't enter our consciousness.
One of these homeostatic processes, and one which is very poorly understood, is maintaining the volume of fatty tissue at a certain level. Since this one has gone freakishly wrong in the modern world, it's the focus of much attention, most of it unscientific and silly: you might even say hysterical. The first thing to get into your head about this process is that, like all the others, it's supposed to just work. You're not supposed to have to think about it and control it from the cerebral cortex. (Which explains, incidentally, why attempts to do so are so pathetically ineffective.)
There are a couple myths to get out of the way here. One is that the volume of fat in the body is supposed to fluctuate: that fat is for long term energy storage. I was taught this in school. You probably were too. It's false. Fat is for short term energy storage. It gets us from one meal to the next. The body has no long-term energy storage: if it did, fat people would starve much more slowly than thin people, and the body would plunder its fat stores before cannibalizing other tissues. It does not, in fact, do this. That's why no one responsible recommends fasting as a diet method. In the absence of food, the body gets right to work consuming its own muscles and bones. We may consider our fatty tissue expendable, but our bodies clearly do not.
There are animals that use fat as long term energy storage, but they are all hibernating animals. Bears, for instance. Most mammals do not, and human beings certainly don't. If you give most animals an abundance of their natural foods, they do not get fat. They get horny. They reproduce a lot. But their body weight stays constant. It you want to fatten animals, as any rancher knows, you have to stuff them artificially with something that will throw their homeostatic processes out of whack. You don't fatten a cow by letting it graze endlessly. A cow allowed all the grass it likes stays obstinately at its goal weight. You fatten it by filling it up with sugars and injecting it with hormones.
The other myth that goes along with this, is that prehistoric human beings used to live in scarcity, on the brink of starvation all the time. Certainly some of them starved some of the time, but it was not the ordinary lot of your ordinary prehistoric human being. It was not until human beings developed agriculture that starvation became a way of life: a good harvest meant plenty, and a bad harvest meant starvation. Before agriculture, food supplies were, by and large, fairly steady and reliable: people were no more likely to be starving in their native habitat than modern chimps are likely to be starving in theirs. It happens, but it's not a way of life. We didn't evolve obesity as a way of coping with food shortages. (Which is good, because it isn't particularly helpful in that.)
We process millions of calories in the course of a year. If we were off by just a percentage point or two we would not be fifty or a hundred pounds overweight. We would be overweight by thousands of pounds. Or underweight by a hundred: that is, dead. This is not a process that is, or was meant to be, under conscious control. I know, I know, people who are normal weight think that they "eat sensibly." If I were normal weight, I'd believe that of myself too. But in fact they eat just like I do: they eat until they feel satisfied, and then they stop. Sometimes they want a treat but decide not to have it, just like I do. Sometimes they decide they want a treat and they have one, just like I do. This has nothing to do with our body weights.
Evidence for this is what happens when you do try to put the cerebral cortex in control of this process. Everyone who's been on a diet knows what happens. You become increasingly irrational and obsessive about food. Eventually you enter a twilight zone of struggle that is completely unlike normal consciousness. And then, sooner or later, you eat whatever you were trying not to eat.
Now, this is not the whole story. There's binge eating to be considered too. But normal-weight people binge as well as fat people. There's no particular reason to think that the homeostatic processes shouldn't be able to deal with binges. The sodium-potassium one does fine: after you gorge on potato chips, you don't die: you just become terribly thirsty, and you take in a lot more water. The body could deal with a massive intake of ice cream by simply making us rather uninterested in food for a week or two.
No, something breaks or overrides the homeostatic feedback systems. That's why we're fat. It has nothing to do with "will power" -- that will o' the wisp of popular superstition. It has to do with blood chemistry and brain chemistry, and what happens when you introduce artificial substances into a body that wasn't designed to handle them.