Imagine that you're walking down the street, when out of the corner of your eye you spot a semi-truck barreling toward you at an astronomical speed. Your instincts kick in and your stress level goes into overdrive. You have to move as fast as you can to get out of the way. For the next few moments, you feel like your life is hanging in the balance.
Now imagine dealing with that feeling when you're casually shopping at the grocery store.