The words, actions and attitudes of people reveal who they are. Jesus was explaining to Nicodemus about the spiritual birth. He compared the life of an unregenerate person to that of a person born again. We all have natures inherited from our parents. We have traits that are a mixture of our parents. These traits distinguish us from one another. We are prone to learn and practice things taught to us by our parents. The nature of sin is something we are born with. It is handed over generation after generation from Adam and Eve. We are prone to commit sin. There is only one way to get rid of sin by believing on Jesus Christ.
Spiritual birth is a stark departure from what we really are. We are born of the spirit. We engage in spiritual things. A deep change happens to a person born again. His words, actions and attitudes speak for themselves. When we compare and contrast both births we conclude, that which is born of the flesh is flesh and that which is born of the Spirit is spirit.