This question may have more than one answer.
There are researchers who have studied this and feel that they can boil it right down to all kinds of enzymes and mechanisms that come into play, stimulating appetite or depressing appetite.
Different foods offer different nutritional scenarios that affect hunger. For example: when one goes and exercises and then sits down afterwards, they need to replenish their water intake. This is something the body needs even though there is no nutritional value to water, per say.
Next, one may become hungry after exercising. A combination of fruits, primarily because they are made up of water, and sugar carbohydrates may satisfy the need for energy after exerting energy. Because the digestive process is pretty rapid when it comes to breaking down sugars into glucose one can consume a lot of fresh fruits and it doesn’t necessarily satisfy hunger.
Hunger pains may still be present because the digestion is completed so rapidly with things such as fruits. However, proteins take longer to digest.
It all comes down to the balance between how many calories your body needs to replace and the calories burned and what kinds of foods you are consuming. It is complex and is another whole area to think about whether it is emotional or whether it is your body actually telling you need nutrition or if it is your body adjusting to less calories.
Researchers studying this issue may have their own theories and their own recommendations. If you have questions about your nutritional requirements, ask your doctor.