Stress is caused by two things. Primarily it is down to whether you think situations around 
you are worthy of anxiety. And then it's down to how your body reacts to your thought 
processes. This instinctive stress response to unexpected events is known as 'fight or flight'.

Flight or flight
The fight or flight response was first noted by one of the early pioneers in stress research, Walter Cannon. In 1932 he established that when an organism experiences a shock or perceives a threat, it quickly releases hormones that help it to survive.

In humans, as in other animals, these hormones help us to run faster and fight harder. They increase heart rate and
blood pressure, delivering more oxygen and blood sugar to power important muscles. They increase sweating in an
effort to cool these muscles, and help them stay efficient. They divert blood away from the skin to the core of our bodies,
reducing blood loss if we are damaged. As well as this, these hormones focus our attention on the threat, to the exclusion
of everything else. All of this significantly improves our ability to survive life-threatening events.

Life-threatening events are not the only ones to trigger this reaction. We experience it almost any time we come across something unexpected or something that frustrates our goals. When the threat is small, our response is small and we often do not notice it among the many other distractions of a stressful situation.