A creature’s (man’s) duty and God’s gift of eternal life are not tied together like a bargain and sale. God gives to the creature and the creature cannot repay God – if a creature could repay God, it suggests that the creature could offer something of equal value and worth.
When God crowns angels and men, He bestows on them purely what is His own and not what is theirs by merit or natural obligation. Indeed, what God gives is given by His gracious obligation to a prior promise which He has made.
From the book “Boundlessly Good”