Let’s face a really trivial question like the reason why you get fat (or the reasons, as we will see later) after losing weight: trivial and brutal, I dare say. In fact, we often think that the reason why we get fat is due to metabolism.
If we have lost weight, we need fewer calories to stay in the new weight. 
In reality, this is true but also too simplified: how many fewer calories are we talking about?

To understand this, just calculate the difference between the total daily calorie requirement we had before losing weight and that of the weight we have achieved . The difference, for a person who loses 2-3 kilos, is about 40 calories, speaking on average. In fact, it has been shown that half a kilo of fat burns two calories a day, and half a kilo of muscle about six. 
Let’s say that with a kilo of weight we lose both fat and muscle: the difference in basal metabolic rate is eight calories on average per kilo. A little more, perhaps if you think that you often lose more muscle than fat, especially if we lose weight quickly or drastically cut carbs or don’t work out.We can then assume ten to twelve calories of variation on the basal metabolic rate per kilo of weight lost. We over-optimize.

So if we lose 10 kilos , without taking into account the initial and final percentage of fat mass, we can say that compared to our starting weight we could eat a maximum of 100/150 calories less, without taking into account the body composition, because we are lowered our basal metabolic rate. In short, the body needs less energy to survive, since we weigh less. 

However, given the all in all modest difference between the previous daily caloric requirement and the new one, this is not the reason why you get fat, given that the variation in daily metabolic requirement due to its major voice, that of the basal metabolism, is not such as to think of gain weight with the maintenance diet, which is a norm-calorie related to your weight loss. So why do we gain weight so easily and lose weight so hard?