Being born again is necessary because all humans are spiritually dead and incapable of entering God's Kingdom without supernatural regeneration (John 3:5).
The necessity of being born again comes from the fact that all of humanity is born spiritually dead due to the fall of Adam. In John 3:5, Jesus states, 'Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God.' This emphasizes the complete inability of man to save himself or even to initiate his own rebirth. It is through God's sovereign grace that He regenerates the hearts of His people, allowing them to respond in faith. Without this sovereign work, no one can have access to salvation or eternal life.
Scripture References:
John 3:5, John 3:3, Romans 3:11
Commentary