Why/how to people cry?

This question counts for two : Why do people cry, and how do people cry (like what triggers tears to come out, and that weird crying face).

Crying is a physical reaction that relieves stress from emotional extremes. It most often causes facial contortion and the tear ducts to secrete. The fluid is different than the secretion caused by eye irritation.

It is triggered by an involuntary reaction to the fifth cranial nerve. An infant who's nervous system has yet to develop, will cry in response to any discomfort. As that nervous system develops, we learn to control the response but when the stress level exceeds the control threshold, the response is triggered automatically and uncontrollably. The facial expression is an indication that the stress level is beyond the threshold of our ability to control it.