What is stress?... Stress is the body's individual response to change. Stressors and stressful situations are not problematic for all people, some people are relaxed by driving and some are stressed, [19] what causes fear for one person, such as rock climbing, may be fun for others.

Stress can be motivational and positive, exercise, a job promotion, purchasing a new home, marriage or birth of a baby, holidays, vacations. This type of stress is positive [13 ]

People avoid feelings of distress and think of this as the bad kind of stress, some examples are excessive work demands, loss of a loved one, car troubles, financial problems.

Though stress can have unpleasant effects, it is an important and useful part of life. It stimulates learning and personal growth, and is a natural part of any important accomplishment. In fact, the most effective and healthiest people are not those who avoid stress,[6] but those who respond successfully to it. [33,37]

The human stress response may be best understood as preparation for exercise under the most severe conditions.
Example: A hiker sees a bear.... In response to seeing the bear a part of the brain called the hypothalmic-pituitary-adrenal (HPA) system is activated, the body prepares to fight or flee. 

The nervous system becomes highly activated and hormonal changes, such as the release of steroid hormones catecholamines, in particular dopamine, norepinephrine, and epinephrine (also called adrenaline), prepare for an emergency response. Muscles tense for action, heart rate and blood pressure increase, breathing becomes rapid and shallow, and digestion and other maintenance functions are put on hold. [Stress Institute]

Click to enlarge

Physiological effects of human stress response are protective and adaptive.
The area of the brain called the amygdala activates an emotional response (fear)

Activities at the front of the brain such as short term memory, concentration, inhibition and rational thought are inhibited to allow for quick response (this reduces skills with social and intellectual tasks)

The area of the brain called the hippocampus stores the emotional experience into long term memory as dangerous stimuli, this would be an evolutionary advantage for avoiding danger (a bear) in the future.

Once the threat has passed and the effect has not been harmful ( the bear has not eaten or seriously wounded the human), the stress hormones return to normal. This is known as the relaxation response. In turn, the body's systems also normalize.

Physical exercise is a form of stress; activating the same
systems as the "fight or flight" response for physical survival. [13]