The fight-or-flight response was first described by Walter Bradford Cannon and is the physiological response that occurs when you encounter a threat, attack, or aggression which often has implications for your survival. This response originates from the autonomic nervous system of which there are two parts: the sympathetic and parasympathetic nervous system. This response should be familiar to most students as it is colloquially known as the 'deer in the headlights.' This reaction is often accompanied by such symptoms as elevated heart rate and breathing, dilated pupils, uncontrollable shaking, tunnel vision, and high stress levels. The fight or flight response is a mechanism that takes over and it is beneficial from an evolutionary perspective.
Imagine a situation where a threat is imposed and you have a minuscule amount of time to decide on a course of action. Making a conscious decision could very well take too much time and so a subconscious and instinctive reaction is the best response. For example, a deer when seeing an approaching wolf will instantly bolt. In fact they often 'take flight' when there is any disturbance or unwarranted sounds. Sometimes 'freeze' is an included reaction to this phenomenon. This can also be viewed as an evolutionarily advantageous response; consider a rabbit who when faced with a threat has the highest chance of survival by not moving and hoping to go unnoticed.