Smiling and stressStressStress is simply a reaction to a stimulus. Stress can disturb our physical or mental equilibrium. In other words, it is an omnipresent part of life.  Stress can negatively impact how we experience our lives by preventing us from living in a state of contentment and fulfillment. Each of us has different thresholds for letting things get to us and we all react (respond) to stress differently. Some people withdraw; some people experience anxiety; some lose their cool.
You may not be able to control the stressors in your world, but you can alter your reaction to them. Stress can actually act as a serious motivator in changing one’s behavior. If you look at stress in a positive light, that it drives the individual to find solutions for problems and to grow, stress can be your friend. Using stress as a source of motivation comes with a sense of urgency in your actions. (?D) Much like running down a hill with a large boulder right behind you, there is little need to dig deep for motivation since you obviously want to alleviate and/or eliminate the anxious feelings.
With a sense of urgency hanging over your head your ability to maintain your focus also increases as well. Now that you are fully concentrating on what it is you are doing you are better able to think more clearly allowing you to improvise more easily. It is amazing that as we get pushed to the ‘edge’ how quickly we can think and respond in certain situations. Hats off to our survival instincts! Now this is not to say we should seek out tension deliberately to serve as a motivator, but that there are certain times such an undesirable can be of service.
Sometimes your joy is the source of your smile, but sometimes your smile can be the source of your joy. — Thich Nhat HanhThe real SmileA smile is a facial expression formed by flexing the muscles near both ends of the mouth and by flexing muscles throughout the mouth.  When you feel happy, the corners of your mouth turn up, your cheeks lift and your eyes crinkle. The overall effect tells the outside world that you’re feeling happy on the inside. It’s simple and, in most cases, totally spontaneous. We typically smile without making a concerted effort to do so.
The free expression by outward signs of an emotion intensifies it (what does it mean ? )Duchenne smileThe appearance of a genuine smile, one involving specific changes in the eyes in addition to the mouth (notably a crinkling of “crow’s feet” and a downturn of the outer points of the eyes) is called a Duchenne smile.  Fake smiles can be performed at will, because the brain signals that create them come from the conscious part of the brain and prompt the zygomaticus major muscles in the cheeks to contract.
These are the muscles that pull the corners of the mouth outwards. Genuine smiles, on the other hand, are generated by the unconscious brain, so are automatic. When people feel pleasure, signals pass through the part of the brain that processes emotion. While facing others, we may use a counterfeit smile to show we are polite. Sometimes we just want to “appear” happy or friendly for social reasons as smiling is important to shaping relationships with others. Smiling and stressWe smile because we are happy, and we frown because we are sad.
But does the causal arrow point in the other direction, too? The facial feedback hypothesis states that facial movement can influence emotional experience. For example, an individual who is forced to smile during a social event will actually come to find the event more of an enjoyable experience. So, smiling speeds recovery from stress. How? Researchers discovered that simply smiling can reduce stress and increase well-being. Even those who were smiling only due to their instructed chopstick position—without explicitly being told to smile—showed the same effect.
Since heart rate is an indicator of the body’s stress response, it seems as though the act of smiling actually reduced the participants’ overall stress level. (should elaborate the heart rate – stress relationship)Each time you smile you throw a little feel-good party in your brain. The act of smiling activates neural messaging that benefits your health and happiness.  The face, in particular, appears to play a big role(what role? Can discuss more on the function of the face). The onnection between facial expressions and underlying mental states is still largely unexplored, but some have suggested that smiling could reduce levels of cortisol, a stress-related hormone.
This flips our traditional understanding of emotion and appearance on its head: Feeling good could sometimes be a consequence of smiling, not just the other way around. When a situation has you feeling stressed or flustered, even the most forced of smiles can genuinely decrease your stress and make you happier. (So here your main point is that “feeling good can be a result of smiling” .. think it is better to refer to the REAL smile and Duchenne smile again in your conclusion to make your essay more cohesive and have better linkage)(Also, you mentioned some “pros” resulted from stress at the beginning, so stress is not absolutely bad to us. The main point is to know how to control it in order to maintain it at a suitable level. Hence I suggest “smiling is a way to control stress, rather than 100% opposing stress. ” You can consider insert this point in your conclusion as well. This can prevent some misunderstanding and contradiction between your “stress part” and “smiling part”.