The sympathetic nervous system is the system of  "action", the "flight or fight" reaction, that mobilizes all bodily functions in situations of stress, i.e. in danger. The parasympathetic system activates all processes, that relax the body, digest the food and regenerate the body. Both systems should work together in harmony for optimum function. We are born with a balanced nervous system, but external conditioning and experiences will shift the balance in one direction. Depending on which system becomes dominant, we then have the typical reactions of a sympathetic or parasymparthetic dominant person.