The set-point theory suggests that body weight is regulated at a predetermined, or preferred, level by a feedback control mechanism.I agree with this, as long as your weight is undisturbed by numerous other factors which can affect its course. In that case your weight is not necessarily within your original pre-determined set point range. It may be above or below.
The way your weight responds to varying stimuli and environments conducive to affecting your metabolism; different forms of stress, mental imbalance/disturbance, grief of all kinds ageing, even things like climate and so on is genetically influenced. And possibly quite individual.
The reason why such a large group of people put on weight is that there are many of these factors and combinations of them, our metabolism is designed to make sure our needs are met making it highly responsive. Yet at the same time it is incredibly secure and regular.
The overall problem I have with a rigid idea of pre-determined set point range is not the idea of it so much as the application of it to suggest that what you weigh now is inevitable destiny it might be, it also may not be. I believe that’s the case only if nothing you are susceptible has acted on it to change it. In a way that has not be reversed or nullified. There's also the interesting way the upward trajectory of our weight seems to have an overall set point range of its own.
If the latter is not the case but the former is, you could be quite far from your weight's original aim and/or in/outside its range. It might be altered by things, on its way there so you might never have been at your original destination.
The theory, which is really an observation of the fact that our weight regulates itself and within a far narrower range than it seems from our end. Obsessed as we've become with every pound, we forget that we eaten and moved around more or less every day of our lives, with relative freedom and yet it has all worked itself out with surprisingly little variance overall.
It doesn't explain why we cannot lose weight and remain that way. That's not directly about set point per se, rebound happens whether you are in your range or not. If anything rebound is a mirror of the self regulation process of weight.
You are programmed to go back to (more or less) where you were when you started the diet because you are programmed to make good that particular loss, regardless of whether that is your pre-destined weight or not. Sometimes if its not, you can find the re-gain stops short or goes beyond if you were below your set point.
Using set point to explain either why we return to our starting weight after a diet, confuses the issue of weight change making it harder to explain anamolies. Its more about why our weight remains constant if there is nothing powerful enough to re-route it either way.
I understand people appreciate the idea that they were meant to be this fat/thin after being made to feel what they weigh is illegitimate, however I don't think it should matter either way. Where you are is where you are. If you live in the sun your skin may darken and the opposite in cold climbs, who cares?
You are still what you are, not defined in solely in terms of what you are not.