Wind and Spirit are often paired in Scripture. Both English words are translated from the same Greek or Hebrew words. Those same words are also sometimes translated “breath”. Spirit, wind, breath. I assume that Jesus was using a familiar earthly concept, wind, to explain an unfamiliar spiritual one.
Those who have been born again, or given new life by the Spirit, have the Spirit of God dwelling in them. They have the resources of the Spirit and can access the mind of the Spirit. They are born into a new way of looking at life.
Jesus compared the ways of the Spirit with the ways of the wind. The wind has no material substance. You don’t know where it comes from or where it’s going. You can’t grab hold of it and feel its texture. You only know wind because you can see its effects.
Suppose, then, that some people decided they do not believe in the wind. They will believe that trees lean over all by themselves or that leaves lying quietly on the ground sometimes suddenly jump up and twirl through the air. They will ascribe power where there is no power. They will not understand that the trees and the leaves are responding to a power that is acting on them.
If a person who does not believe in the wind and a person who does look at the same scene, they will see two startlingly different realities. The first will see trees bending over; the second will see the wind blowing. That person will not be limited to time-bound, earthbound perceptions and short signed vision. He or she will also recognize the work of the wind.