The primary reason for this law was to stop people from doing additional harm to one another in circumstances where a marriage has broken down. As a result of the hard and sinful hearts of human beings, this command was given. Hardness of heart refers to human stubbornness (Deuteronomy 10:16; Jeremiah 4:4) and sin as the cause of divorce.1
5 And Jesus said to them, “Because of your hardness of heart he wrote you this commandment.