The phenomenon, which we now call stress, is actually a useful and very important system for the preservation of human life. In a time, in which many threats were of a physical nature and existential necessities could quickly arise (namely survive or die) it was very important for man, that he reacts in a certain way to a crisis situation.
If, for example, one of our ancestors in the early days of mankind, who basically had the same physical stress response as we do today, met an animal enemy, it was very important to him, that he switched his physical responses to fight or flight. The energies of the body were collected and bundled, to make them available to the muscles, which could prove useful in escaping or fighting.
The reactions, that made sense in a crisis at a time like this, are pretty easy: the vegetative nervous system is activated, digestion shuts down, and the muscles become tense. The release of adrenaline makes the human body more efficient and enables it, cope better with physical confrontations.
For example, if in the early history of mankind a representative of our species encountered a predator, which threatened his life, he could use the stress reaction in this encounter to perform better and thus increase his chance of escaping or winning the fight.
This makes an evolutionary lore of this reaction very useful. The problem, that we have to deal with today, is the simple fact, that this reaction is mostly no longer appropriate in today's world. Problems, with which we are confronted in our everyday life usually no longer threaten our lives, but usually only certain feelings or at best our finances. However, our body does not know this difference, because evolution is much slower, as humanity evolved - our genetic makeup, who are responsible for the structure of our body and thus its functioning had in the approximately 100.000 Years, in which our species consists, no chance, to adapt to the new challenges.
Stress is thus an evolutionary anachronism, which is due to the rapid and special development of our species.
When an animal has to deal with such a rapid and fundamental change in its habitat, as man has done over the last millennia, it will most likely die out. Humans, on the other hand, are very well able to adapt to such environments because of their social and intellectual achievements, which no longer have much in common with the physical level of our existence - it is actually unthinkable for our body, that we earn our living, for example, by designing clothes or producing steel girders, because these products have nothing to do with our direct bodily needs. However, the human being as a whole system is usually able to adapt to such changes quite well, precisely because, in addition to his physical reactions, he also brings a mental level with him, which is very flexible. Therefore, the inappropriate response "stress" is not a threat to our species, because our ability to adapt is far greater than that of any other species.
Yet we suffer from inappropriate and no longer appropriate physical responses, because they are offset by our possibilities for compensation, but cannot be undone.
A good example of such a reaction is, for example, in addition to stress, pain. Humans certainly have the mental abilities, recognize dangers to their bodies, without this having to be accompanied by pain - we could certainly grasp it through psychological learning processes, that a hot stovetop is not the right place for our hand, even if we didn't feel any pain. The fact that we do it anyway is also based on an evolution, which cannot grasp the scope of a person's mental capacity at all - so there is a whole range of physical reactions, which today seem out of place and hardly make sense.