Think of the last time you played sport, or even were involved in an intense game of trivia down at the pub. Your heart is beating faster, your blood pressure is higher. You really need to win — even if you won't admit it to yourself (especially when it comes to the trivia). Game over, you come home victorious. Don't you feel great? You have been flying by the seat of your pants, making quick decisions, razor-sharp thinking …
And yet you have also been quite stressed.
The brain's primary function is to keep us safe. If danger looms we react fast. When we feel safe, we have the luxury of taking the slower thinking route involving the prefrontal cortex to consider our response.
Acting on reflex denies us the time required to consider all options and make the best decisions. As a result, we tend to believe it's our emotions that determine how we react at any given moment. The reality is in fact the opposite: we rely on our emotional regulation to ensure we respond appropriately.