In short, the Enlightenment privatized marriage, taking it out of the public sphere, and redefined its purpose as individual gratification, not any 'broader good' such as reflecting God's nature, producing character, or raising children. Slowly but surely, this newer understanding of the meaning of marriage has displaced the older ones in Western culture.
Timothy J. Keller, The Meaning of Marriage: Facing the Complexities of Commitment with the Wisdom of God
love, marriage, the-enlightenment