Why is marriage indissoluble?
Marriage is triply indissoluble: first, because the essence of love is mutual self-giving without reservation; second, because it is an image of God's unconditional faithfulness to his creation; and third, because it represents Christ's devotion to his Church, even unto death on the Cross.
At a time when 50 percent of marriages in many places end in divorce, every marriage that lasts is a great sign - ultimately a sign for God. On this earth, where so much is relative, people ought to believe in God, who alone is absolute. That is why everything that is not relative is so important: someone who speaks the truth absolutely or is absolutely loyal. Absolute fidelity in marriage is not so much a human achievement as it is a testimony to the faithfulness of God, who is there even when we betray or forget him in so many ways. To be married in the Church means to rely more on God's help than on one's own resources of love.
What threatens marriages?
What really threatens marriages is sin; what renews them is forgiveness; what makes them strong is prayer and trust in God's presence.
Conflict between men and women, which sometimes reaches the point of mutual hatred in marriages, of all places, is not a sign that the sexes are incompatible; nor is there such a thing as a genetic disposition to infidelity or some special psychological disability for lifelong commitments. Many marriages, however, are endangered by a lack of communication and consideration. Then there are economic and societal problems. The decisive role is played by the reality of sin: envy, love of power, a tendency to quarrel, lust, infidelity, and other destructive forces. That is why forgiveness and reconciliation, in confession as well, is an essential part of every marriage.
(From the Catholic Youth Catechism questions 263-264)