From the point of view of physiology, this is an example of the so-called. generalization of excitement. Those. a focus of excitation that has arisen in one part of the brain is transmitted to others, often and not particularly connected with each other (this is called irradiation). Another example can be given this: a person frowns and groans with severe pain.
By the way, generalization in children is much stronger than in adults. Observe a child focused on some work, for example, collecting a pyramid. How he "helps" himself with facial expressions! He can do something with one hand, while the other hand will repeat the movements, although it is not busy. A tongue sticking out from zeal is also an example of generalization of excitement. With age, the brain already learns to localize arousal processes, but not in all cases.
Just as screaming or swearing helps relieve stress (remember at least the same weightlifters) - facial expressions in this case have several functions.
The first is the same (maybe imaginary) relief: " Here I am! I am strong! I am trying! "
Second, according to the logic of things, a person, as a social being, can thus give a sign to others a la" it's hard for me, I need help "