Why do people need both the sympathetic and parasympathetic systems to maintain physiological balance and respond effectively to different stressors and relaxation cues?

The sympathetic and parasympathetic systems work together to regulate the body's response to stress and relaxation. The sympathetic system activates the body's fight-or-flight response during stress, while the parasympathetic system helps the body relax and recover. Both systems are needed to maintain balance and respond effectively to different situations.