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.
Too restrictive - Many conventional diets demand a low calorie intake in order to lose weight. For people who have a large amount of weight to lose, reducing their usual daily intake by 1,000-2,000 calories a day is a depressing task. Such dieters feel deprived before even starting a new diet. The amounts of food seem very small and usually include uninteresting foods such as yogurt, cottage cheese and chicken breasts.
Requiring a major change - Humans are creatures of habit and usually eat the same foods repeatedly. However, in an effort to promote eating a variety of healthy foods, conventional diets suggest new dishes, which often include exotic or just plain boring foods. Using a sample week's menu of meals can result in buying unusual ingredients, using a small amount for one recipe, then often wasting the rest.
Difficult to follow - Most diets suggest using fresh foods, cooked from scratch at home. This requires more meal planning, shopping and preparation time. It's easier and quicker to rely on fast food or convenience foods. The drawback with fast food is in controlling exactly what is eaten since the ingredients are not easily known. New recipes can take longer to prepare, making it tempting to revert to old eating patterns and simply give up.
Feeling like a punishment - Diets require the reduced intake of food, cutting out favorite foods, learning to like new foods, and spending more time planning and preparing food. All these changes can make the dieter feel punished by the very process. People usually approach a diet with the attitude: 'this is just until I lose x number pounds.' This is where people fail diets. Any change required to lose weight will need to continue after the pounds are gone. When dieters revert to old habits, the weight creeps back on.