When there is a stress coming from the physical or psychological environment, the body responds with a stress reaction. One cannot have stress but one can be stressed and one can have a stress reaction. Stress reactions have evolved over hundreds of thousands of years to defend the individual from physical dangers. They make good sense as survival mechanisms against physical stresses for short periods of time, for instance to escape an attacking animal. They can be seen as the body temporarily re-prioritising its resources. For example, blood supply is diverted from the periphery to the muscles. This reduces blood loss in the case of injury, and increases strength. Unfortunately the body responds to psychological dangers as if they were physical. Our social environment has evolved faster than our biology which continues to interpret and respond to psychological stresses as if they were a physical attack. The problem is further complicated by the fact that psychological stresses are often of long duration compared to being attacked by a hungry animal.
One can draw an analogy to a jet airplane at takeoff. The jet engines are provided with after-burners that give an extra boost to get the plane off the ground. At takeoff, large quantities of fuel are poured into the after burners for a burst of power. But this trick cannot be sustained indefinitely because of overheating and inefficient use of fuel.
STRESS: Sabretooth Tiger Relevant Emergency Survival Strategy