Why is rejection of God's pardon of our sin a sin? What is sinful about that?
Why do we need pardon for behavior which is an unavoidable part of our nature?
Adam was our forefather who had dominion over the entire earth. He forfeited that dominion and brought on the curse when he chose to disobey the Creator's direct commandment. Basically God said this is all yours, no sweat, if you do not eat of the tree of the knowledge of good and evil. After the fall life became the grind that we recognize as the rat-race we live with today. Continually having to overcome the natural way things become disorganized.
God created mankind to rule the earth for Him. In our fallen state we relinquished the potential He had designed into us. By rejecting the pardon, we can no longer have fellowship with God because evil is not in God's character.
Being a loving Creator He has provided a remedy that costs nothing for us. With physical death we are either ushered into His loving presence to recapture our original inheritance, or we are placed in a place where none of His goodness will reach. No more breezes to cool us down. No more sunsets to inspire the feelings of personal value for life. No more love and camaraderie. All of these things are outcomes of His love for us, and the reason we have no excuse for rejecting Him.