John 3:3 Jesus answered and said to him, Truly, truly, I say to you, Unless one is born anew, he cannot see the kingdom of God. 5 Jesus answered, Truly, truly, I say to you, Unless one is born of water and the Spirit, he cannot enter into the kingdom of God.
A kingdom is always related to life. The vegetable kingdom is related to the vegetable life, and the animal kingdom is related to the animal life. If you want to share in a certain kind of kingdom, you first need the life of that kingdom. Only men can participate in the human kingdom because only they have a human life. So, without the life of God, how could we ever share the kingdom of God?
The kingdom of God is not only the reign of God, but also the divine realm or sphere. In order to participate in the reign of God and to be in the divine realm, we need the divine life of God. Only those who have the divine life can be in the divine realm and share the divine kingdom. Thus, we need to be regenerated that we may have the divine life which enables us to enter into the divine realm and participate in the divine kingdom. Even if we were not fallen or sinful, we would still need to be born again, because regardless of how good, pure, and clean our human life might be, it is not able to realize the things of the divine realm and it is not qualified to enter into the divine kingdom. Only the divine life is qualified to be in the divine realm. Only the life of God meets the requirements of the kingdom of God.