Back when I was in college, I took a course in experimental psychology. As a part of the class, we had to do classic experiments with white rats, Skinner boxes, and mazes. For those who are blissfully ignorant in the jargon of psychology, "Skinner boxes" are little cages which have a bar that a rat can press to dispense a food pellet.
Many people know the basic details of these types of experiment, but they don't know what happens to the rats in order to encourage learning. Rats, like people, are lazy and will not feel motivated to learn or change without a compelling reason. In order to get the rats to care enough about the food pellet to go to the effort of searching the cage for food and stumbling on the fact that the bar relates to getting food, you have to make them pretty hungry. Without deprivation, the rat is indifferent to getting the food pellet and will just sit around in the box being happy and contemplating rat philosophy.
The rats that are trained in psychology classes are doomed to a short existence. After they have been trained, they are rendered useless and are usually destroyed. I was told, perhaps seriously, perhaps in jest, that the way this was done was to break their necks with a flick of the wrist. Since state colleges don't have a lot of funds or veterinary expertise, it is unlikely that they are chemically euthanized. When I was told this about my rat, who has taught me so much about life that I think she deserves a better name and memory than the one I tend to give her (I named her "Rat Rat"), I chose to adopt her as a pet rather than to have her head twisted around until she expired.
My rat, who I was actually not really fond of as a pet but merely saved out of humanistic impulse, was not a normal rat after the conditioning she went through. Because she was sometimes deprived of food in order to compel her to learn, she would eat as much as she could all of the time. Since I was no expert on rat health, I just fed her whenever it seemed she was hungry. Soon, she grew very fat. I tried to feed her less, but she seemed to become anxious and paced the cage nervously looking for food when her food was reduced.
Within about a year, my rat died. She wasn't very old even by rat standards, probably no more than one and a half years old. I sometimes pondered if I fed her to death or if she simply was not healthy because she was the result of too much white rat inbreeding. My professor told us that, to save money, he had to keep breeding the same pool of experimental rats until he felt it was no longer an option because fresh rats were terribly expensive. I'm guessing she may have suffered from both too much food and her bad genetic pedigree.
My rat's situation teaches some valuable lessons about behavior that can be applied to humans. As I mentioned in the linked post a few paragraphs back, she showed the power of habit, routine, and superstition. In the case I'm currently talking about, she shows the strong effect of deprivation. Had she not been made terribly hungry in order to motivate her to learn in the Skinner box, she probably would not have become so fat later. She also likely would not have grown so anxious at any food reduction. If this is starting to sound familiar, it is because it is a classic situation that human dieters go through.
Dieting requires food deprivation over an often prolonged period of time. The more severe the deprivation, the more likely one will be to develop psychological issues with food and a disordered relationship with it. That is not to say that people can't practice caloric reduction without resulting food issues, but there is little anecdotal evidence to support the idea that they don't and ample to say that they do. Just like with my rat, food deprivation fuels anxiety about eating and not eating. It encourages overeating, and creates irrational feelings about food such as attaching moral implications to the choice to eat certain things.
Because of my understanding of these issues, I decided early on not to decide that food or eating were "good" or "bad" and not to put any food out of bounds for me. While I certainly have "guideposts" that I would like to remain in, I don't get worked up if I step beyond the boundaries. I just try every day to roughly stay within them. I view choices broadly as moving in a direction I'd like them to or in one I would prefer they not go in. I realize that making a particular choice slows my progress while making another keeps it up. It's not the end of the world if I doddle a bit on the road by eating a few hundred more calories than I view as optimal.
It is perhaps a very difficult thing for people to accept that the greatest restriction they can possibly endure is not the best path to weight loss. You can explain again and again that there are complex psychological reactions at play and they will be rejected out of hand as "excuses". It generally takes multiple failures before people even begin to consider that they may not be able to tread a severe path without consequence. My poor rat didn't choose to not eat for a few days before training nor to only be able to eat what she was smart enough to earn, but we humans have the capacity and the power to do better by ourselves. Whether or not we choose to do so is an entirely different issue.