84 Assuredly, also, will there be wanting that close union of spirit which as it is the sign and mark of the Church of Christ, so also should be the sign of Christian wedlock, its glory and adornment. For, where there exists diversity of mind, truth and feeling, the bond of union of mind and heart is wont to be broken, or at least weakened. From this comes the danger lest the love of man and wife grow cold and the peace and happiness of family life, resting as it does on the union of hearts, be destroyed. Many centuries ago indeed, the old Roman law had proclaimed: "Marriages are the union of male and female, a sharing of life and the communication of divine and human rights." But especially, as We have pointed out, Venerable Brethren, the daily increasing facility of divorce is an obstacle to the restoration of marriage to that state of perfection which the divine Redeemer willed it should possess.
Source: Casti Connubii (Vatican.va)