The Stress response mechanism evolved on the plains of Africa tens of millions of years ago. Its purpose back then was quite simple, to keep us alive, to protect us from predators whose goal was to kill and eat us.
Due to this most primitive function, self preservation, the way our bodies are effected by the Stress response are extreme. When our ancestors on the plains of Africa were in mortal danger, the basics to survival were all that mattered. After an encounter on the plains, their was one of two possible outcomes. Either the encounter was over with, and you survived, or you were over with.
When our ancestors came face to face with death, this automatically triggered the release of massive quantities of two hormones from our adrenal gland. Adrenaline and Glucocorticoid.