I have been born again many times
Like Nicodemus, many people have no concept of what it means to be born again. He thought Jesus was speaking of a physical rebirth. Others see the experience as being a spiritual "tingle" when they think of God or a warm fuzzy feeling when they enter a building they erroneously call a "Church." Or maybe they are of the impression that one is born again when one is "christened" or "confirmed."
However, the new birth spoken of by Jesus is absolutely essential for sinners to enter heaven. If they are not born again, they will not enter the kingdom of God. Therefore it is necessary to establish the fact that one becomes a Christian by being born again, pointing out that Jesus Himself said that the experience was crucial.
The difference between believing in Jesus and being born again is like believing in a parachute, and putting one on. The difference will be seen when you jump. (See Romans 13:14.)
How is one born again?
Please read page titled What is the Gospel? under the questions section.