Just as there is nothing contrary to God but sin (for devils are not so but by sin), so sin in being contrary to God, is and cannot but be contrary to man.
Inevitably, that must be evil to man, which is evil against God, who is the chiefest good of man. Communion with, and conformity to God is man’s felicity, his heaven upon earth and in heaven too, without which it would not be worth his while to have a being. Now since sin is a separation between God and man, an interruption of this communion and conformity, it must needs be prejudicial and hurtful to him.
Besides, the commandment of which sin is a transgression was given not only for God’s sake, that he might have glory from man’s obedience, but for man’s sake, that man might enjoy the good and benefit of his obedience, and find that in keeping the commands of God there is great reward.
The Sinfulness of Sin, 37.