Answer: They have very small minds. They feel only their feelings and cannot balance them with reason. That is why they are either happy or they cry.
Question: Does this phenomenon have a spiritual root?
Answer: This is a very small Kli (vessel), which lacks reasoning ability. That is why a newborn displays what it feels.
My Comment: Among adults there are people with different sensitivities. That is why their tears are not related to spirituality. Kabbalists are prone to serious expressions of emotion, but these, as a rule, are rarely seen externally.
From the Kabbalah Lesson in Russian, 12/31/17