If we were to look for two culprits, they would be overactivity of a facial muscle on the one hand and the aging process on the other. Muscle overactivity is responsible, especially in young people, for the corners of the mouth being pulled down sharply or deep marionette lines appearing at the sides of them when laughing heartily. It is called the musculus depressor anguli oris. But in order for us to look happy, there are still counterparts on the face that pull up the corners of the mouth. These are several fine muscle groups with again long Latin names. If there is now unconscious overactivity of the depressor muscle, the antagonists cannot keep the wound angles in balance and a fold is formed.
On the other hand, facial fat pads migrate from the top to the bottom as we age. That is why the face often looks heavy in older people. As the upper half of the face looks rather bony especially in the temple and eye region and rather oily in the lower third. The consequences are an increase in static facial wrinkles such as nasolabial folds, marionette lines and sagging cheeks.