Some people seem to eat whatever they want and never gain a pound. The rest of us have to be really careful. The reason for this is metabolism: the mechanism by which your body burns the calories you eat and converts them to the energy necessary for your body to function.
There are a lot of myths about dieting and ours is a world where it seems everyone is on, just finished, or about to start, a diet.
The only really successful way to become slim is to change your lifestyle: to do more exercise and eat well.