It’s a natural response to enable you to react to a potential threat.
This is your body’s natural response to a perceived threat. In our evolutionary history stress was caused by a potential life-or-death situation like an approaching stranger or predator, so survival depended upon your ability to fight, or run away - the ‘fight or flight’ response. The amygdala (the instinctive part of your brain) responds to the perception of a threat by flooding the body with cortisol and adrenaline to enable you to react quickly, and releasing glucose into the blood to give you the fast-access energy you need.
Unfortunately, amygdalas can’t tell the difference between a sabre-toothed tiger, an angry boss and a difficult customer, so reactions to any of these modern-day stresses is the same as if our lives were in immediate danger.