Why is the nervous system the most important system in our body?
Because it regulates every process in the body.
The nervous system is the complete network of nerve cells (neurons). It includes the brain, the spinal cord, the nerves and all the sensory neurons in the body.
In short the nervous system is what makes us human. It allows us to feel, think, act, live, love etc.
Sensing and acting
The sensory neurons gather input such as temperature, body position, pain, light, hunger etc. This information is transmitted through the nerves to the brain where the information is processed. This processing means interpreting what to do with the information. When this is decided a signal can be send through the nerves to generate output such as contraction of a muscle when your hand touches a hot surface.
Control of all bodily processes
Next to sensing and acting, the nervous system controls all processes in the body. It can stimulate tissues to produce hormones and enzymes. It stimulates the involuntary movement of muscles such as the heart and the bowels.
Movement and emotion
Not unimportant is that the nervous system allows you to move your body by voluntary control of the muscles. As part of the nervous system, the brain is of course extremely important in generating emotions, allowing you to enjoy, love, be angry and sad. The brain enables learning, memorizing, thinking, dreaming, control of behavior etc etc etc.