Except in our world, fight or flight doesn't really make sense because we don't really face very many situations in which either choice is really warranted (thank goodness!). If your boss corners you in the office and reads you the riot act about something or other, you're stuck. A physical fight response will get you fired (though many people go there anyway) and it would look kind of silly to scream and run away as fast as you can. So we freeze. Tongue tied and tense, we're riveted to the spot. Afterward, when blood flow returns to normal, we think of all the great things we could have said in that moment, or all the ways we could have handled it differently. Cause those things live in our prefrontal cortex.
Over time, your brain will develop the attitude that it CAN handle any situation rather than it CAN'T, and sensory input doesn't trigger an amygdala response nearly as often. Ta DAH!!!! What??? Handle STRESS without ANXIETY???? No way.