Catholic belief hinges on the person's will in alignment with God's will. God's will is known through prayer, commandments He has already given and the hierarchy He established at Pentecost in His apostles and with the Holy Spirit.
Marriage within the Catholic Church emulates these beliefs; it exemplifies them at their best. It is the marriage of three wills, not two. A man decides (wills) to marry and asks a woman her will to marry him; together they ask God's will for them. They (their wills) most be free (no other promises made) to marry each other. This same respect between the three does not stop at the wedding day. The couple's acceptance of God (and His Church) within their daily decisions is something everyone forgets about, but can immensely help the survival of the marriage. "Who is like God" was St. Micheal's cry against the devil.
I hope this helps.