In her book "Rethinking Thin", Gina Kolata recounts the mental struggles of an overweight dieter as he tries to come to terms with the fact that not only will he have to avoid many of his favorite foods while he is actively losing weight, but that he will also have to severely limit them for the rest of his life. The paradox for him is that "he knows from his own experience that it is possible to refuse food, to be totally uninterested in it, for religious reasons." This man has kept a strict kosher home for the last seventeen years, and even though he remembers how much he used to enjoy lobster and crab cakes, he is not going to eat them now no matter the circumstance because he simply "does not eat that food" and it takes essentially no willpower to avoid them. In spite of the ease with which he shuns food for spiritual reasons, he has a very difficult time shunning food for corporeal reasons. His desire to be thin, he has come to realize, isn't as powerful as his desire to be pious.
Which leads logically, I think, to this question: could it be that in order for some individuals to adhere to a diet, they have to perceive it as if it were something akin to a religion? Having to restrict food, especially for a lifetime, can be a rather depressing thought. As Gina Kolata's dieter asks: "How do you get the reasons why you want to lose weight to be important enough to maintain self-control forever?" Perhaps some people accomplish it by developing the mindset that their particular diet is "The Way". By conflating their way of eating with righteousness and ultimate truth, they are now, in essence, limiting food for religious, not merely run-of-the-mill, reasons. And for some folks apparently, that's a lot easier to do.
Although it can help people stick to a diet, this interesting little mind trick is not without its downside. Dietary True Believers emerge; it just goes with the territory. You know the type: they are convinced that their diet is the best, not only for themselves but for everyone; they assert that anyone not doing well on the diet must be doing something wrong; they become hostile when confronted with criticism of the diet and will rarely, if ever, concede a credible opposing point; they will cite science when it is to their advantage to do so, but will dismiss it as flawed and biased when it is not; and they oftentimes regard only a few sources as truly authoritative and trustworthy, e.g., a particular book or "guru". But most perplexing of all, they sometimes will not even recognize when the diet isn't working for themselves.
Yes, changing one's eating habits is often difficult. But the "diet as religion" route is not the way to go. Too much risk involved in my humble opinion.