On an evolutionary level your stress hormones are designed to help you run away from a sabre-tooth tiger or turn around and fight it, if you decided to!  This is called the flight or fight response.

Today thankfully we don’t usually have to run away from sabre tooth tigers or any other scary creature, but we also don’t have long stretches of time between stressful life-and-death events like we would have in our ancestral past; relaxing in caves, walking in nature, and singing together around a fire.  Instead of these little spurts of high-stress, we now have relentless medium grade stress every day.