When you feel stress, your body is preparing to fight something or run away. The fight or flight response involves your brain, your body, and your emotions. It starts in a part of your brain called the hypothalamus. When you feel stressed, you are feeling the effects of your body's preparation to cope with a life-threatening situation. Your body is preparing to fight something or to run away.
Evolution comes at a price.
As our body has evolved, so too has our ability to imagine all sorts of life-threatening situations around us when there are rarely any. As time passed, our brains evolved and we learned how to conquer those things that used to threaten our lives or we learned to better protect ourselves with weapons and technology. But evolution came at a price. As our brains continued to evolve so too did our ability to think in ever more complex ways. As a consequence, we can imagine all sorts of life threatening situations around us when there are rarely any.