If you reduce your calorie intake, by eating less food, or foods that contain less energy, you will lose weight initially, as your body draws on its reserves of fat. But your body will also compensate for the reduction of energy input, and your metabolism will slow down. Then as soon as you relax your diet, all the weight you've just lost will go back on again - and possibly more - because your metabolism is still running too slow.
Calorie counting diets don't often deliver the promised results. It's not the quantity of food that causes weight gain and ill health, it's eating the wrong things, and/or being deficient in something that's vital for efficient metabolism.
Metabolism is the process which extracts the nutrients from your food, for energy and other body functions, for example, keeping bones and teeth healthy. If your diet contains the correct balance of essential nutrients, your metabolism will work at its optimum point - your weight will be right for your height and build, and you will be fit and healthy.
People have different metabolic rates. Some are fast metabolisers, others are slow. Foods that cause weight gain in people with a fast metabolism are not the same as those that cause weight gain in people with a slow metabolism. So there can't be just the one diet that's suitable for everyone.