Strictly speaking, diets don't fail, people fail to stick with a diet. Following any reduced calorie diet will result in weight loss. The problem is sticking with it. Unfortunately, most diets have built-in failures which trip up the dieter.
Diets go wrong by being too restrictive.
Many conventional diets demand a fairly low calorie intake in order to lose weight. Although true, for people who have a large amount of weight to lose, reducing their usual daily intake by 1000 - 2000 calories a day is a depressing task. Such dieters feel deprived before even starting a new diet. Even for people with small amounts to lose, cutting their usual intake from 2200 or 2500 to 1200 calories, can be a shock to the system. A quick glance at any women's magazine reveals at least one sample menu for weight loss.