Here is how I think it happens:

Your body assumes that you may be living in an environment where food could be seasonal, so it stores some fat to protect you against future food shortage.

You try to improve your fitness with your first diet. You reduce your food intake alittle. At first, your body is not sure whether this was due to food shortage or because you are too lazy to look for food. So it gives you hunger pain and dizziness in order to force you to look for food.

You decide to live the pain and stay without eating. Now your body is sure that this was your first food shortage. It uses some of the stored fat to make you survive.

After you reach the weight you like, you stop the diet. Your body sees that the food shortage season is over, however it believes that the danger is now more real. So, it determines to store more fat for you. It forces you to overeat until it stores the larger amount of fat which it assumes you now need.

This cycle keeps going on and on. Each time you try to lose what you gained, you end getting heavier.

Sam