Short answer: Parasympathetic promotes longevity

Sympathetic means stress, and parasympathetic means recovery.

When in sympathetic state, your body enters into a fight or flight response, which enable you to survive from a dangerous event. Sympathetic is useful, but the problem with sympathetic is our body doesn't yet fully adapted to the rapid changes of our environment. Most of the trivial problems can be solved with parasympathetic system instead of putting stress constantly on our body.

Some people may argue that stress is good for adaptation to happen. This is good if you are not in sympathetic state constantly, especially during your sleep, which a lot of people are having nowadays. You need to be able to control and switch between the state consciously, within your will power, and to do that, you need to recover from sympathetic and enter the parasympathetic state for relaxation.

How to enter parasympathetic state

The major problem people are suffering today is over thinking. Thinking requires a lot of energy, which is considered a stress and sympathetic pathway if you are not to able to control it properly. To be able to control your thinking, you need to know the root of your thinking. The root of thinking is to either survive better or reproduce.

This is contradicting because you think in order to survive better, and to survive better, you don't overthink. The keyword here is over in the word overthinking. Thinking is indeed necessity for our survival, but people nowadays is bombarded with just too much information and to do list which our body yet has enough time to adapt and evolve.

To learn to cope with all the stress coming from the overwhelming information and tasks, you need to consciously remind yourself to relax. Meditation is the prefer way to do so. Meditation is a broad term which a lot of people slightly want to run away from due to the default state of mind, which is to keep doing stuffs and thinking for our survival.

Meditation in this case, which is to relax your body is simple. Just sit comfortably (can be on your couch), scan and observe your body, and kindly ask your body to relax, part by part, from head to toe for vice versa. You have to keep repeating this consciously, with more effort in the beginning, and slowly you will be able to do this much easily after your subconscious mind has adapted to your habit (repeated behaviour).