He reminded his listeners of God's creation plan for marriage, and he told them that the reason people divorce is because their hearts are hard. Our Lord's words here deserve some consideration on our part. Both the Old and New Testament allow for divorce. It is not the unforgivable sin. There is honest recognition in Jesus' words that marriages are often tragic.
Marriages create intimacy, and intimacy carries with it great power.
The person who can bless you the most can also hurt you the most. In a marriage where there is hardness of heart, the possibility for pain is very great. Lives become intertwined when there is physical closeness, emotional joining, shared living space, common economic responsibilities, parenting together, for better or worse, in sickness and health, for richer or poorer. Hardness of heart is a core of selfishness, a refusal to listen and understand. It is stunted emotions, feelings, and appetites that make unreasonable demands.