If you are in a stressful situation it is likely that you are not a very happy person this is a natural situation when you are stressed. Whenever you feel stressed the body is primed with hormones which are strongly mood altering. Imagine that a dangerous animal threatens you, your body pumps in adrenaline, to prepare the body for fight or flight. There is an immediate and powerful mood change, you don't start smiling or laughing in these circumstances. Your body is primed for action, and is reacting with the fight emotions of anger, aggression and hostility or the flight emotions of fear, anxiety and nervousness. .
In a natural situation there would have been physical activity in fighting or running and the danger would have passed quickly. In modern society the body response is the same but the environment has changed. The main threats that we perceive are of a financial, emotional, mental and social nature. These types of threats are constantly present in our environment and are not generally dealt with quickly. So, people are constantly primed with the fight or flight hormones and the emotions that are produced by this. This produces a lot of very unhappy nervous, fearful or unhappy aggressive, hostile and angry people. .
Also, this body response speeds up the body's metabolism and so quickly burns up precious reserves of nutrients and energy. So, if this continues less resources are available for you to deal with life and the body starts to degenerate, compounding the situation and making you feel even worse. .
Imagine that there is a secretary in an office and her boss comes in. He is angry, he goes to the secretary and rants and raves at her for no reason. Her body adrenaline cycle is activated for flight or fight, but because of social circumstances she is restrained from action. She would probably like to walk out, but because of family commitments and bills, she sits there and stews.