What God has joined together, let no one separate.

Mark 10:1-12
It was because your heart were hard that Moses wrote you this law. But at the beginning of creation God made them male and female. For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh. Therefor what God has joined together, let no one separate.
Anyone who divorces his wife and marries another woman commits adultery against her. An if she divorces her husband and marries another man, she commits adultery.
Jesus leaves Galilee and travels south. He spends some time in Perea, on the east side of the River Jordan before making his way to Jerusalem via Jericho.
The New Testament Letters may throw light on the arrangement of this chapter. Paul, for example, similarly brings together advice for husbands and wives, children, master and slave, leaders.
Why?
It was because your heart were hard

This passage shows that like divorce, God also hates violence in marriage.  Is it divorce or violence that violates the marriage covenant?  This scripture implies that both do. Certainly domestic violence and abuse are “hardness of heart” that violate marriage vows to honor and protect.  Though God does not want divorce, this may not mean that it isn’t necessary in some cases. God, Himself, divorced the northern tribes of Israel (Jeremiah 3:8), and the priest Ezra insisted that Israelite men put away their pagan wives and children

It is a covenant among couple before the Lord, God hate divorce. Marriage is a lesson to each other.
How?
Respect the covenant of marriage before the Lord, what God has joined together, let no one separate.