“What God has joined together, no human being must separate.” (Mark 10:9)

First of all, humans are created according to the image of God, “God created man in his image.” (Genesis 1:27)
Therefore, humans have shared the life of the Holy Trinity. That is the life of mutual love.

At the same time, women originated from the bone and flesh of man, “The LORD God then built up into a woman the rib that he had taken from the man. When he brought her to the man, the man said: "This one, at last, is bone of my bones and flesh of my flesh; This one shall be called 'woman,' for out of 'her man' this one has been taken."” (Genesis 2:22-23)

Therefore, this intimate the relationship (bone and flesh) of husband and wife has become the power of a single body. For this reason, Jesus added, “What God has joined together, no human being must separate.” True love lasts for eternity!
Paul said, “In any case, each one of you should love his wife as himself, and the wife should respect her husband.” (Ephesians 5:33)

It is simply because, “For no one hates his own flesh but rather nourishes and cherishes it, even as Christ does the church, because we are members of his body.” (Ephesians 5:29)

If we do not base on Christ as our “root”, we naturally cannot love deep enough.