Many people go on a diet and fail.
The most probable explanation is that many diet plans are based on calorie counting or depriving yourself of food. These diets fail for the simple reason that people get hungry and revert back to their former eating pattern. A good diet should not be based on deprivation you should be able to eat as much as you like of delicious filling food and never feel hungry. A high fibre diet where the main source of energy is complex carbohydrates will satisfy your hunger. There is a vast choice of healthy cuisine available that tastes great.