Thirty years of life affords one to see a fair number of weddings, to know friends' parents and to see friends themselves wed, which in our secular days means I have also been given the chance to see many of those unions fall apart. The reasons are only occasionally infidelity. More often than not they are people who did not grasp the gravity of what they were doing in the first place.
We blame the sexual revolution for destabilizing marriage. It certainly destabilized meaningful acts of intimacy between two people and made them into objects of their own value, but Al Bundy, Ray Romano, Sam Kinison, and '90s chick shows did more damage to marriage by presenting the institution either as an apotheosis of perfect romantic sentiment for women and as a quotidian trap to bore men.
Why marriages fail are unique to each one, but what is strange among people are the tales they tell themselves to exempt themselves from the voice of their own consciences. Some people are simply ill-suited for marriage. Idealists belong either in monasteries or universities. Fathers, either of families or parishes, must be grounded people with firm backing of principles. However, some divorcees do not wish to learn this about themselves and move on.
And then there are the mental and religious calisthenics that follow. Most recently I witnessed a 25 year union that produced three children end. The father was an atheist, the mother a daily Mass Catholic. He remarried to a woman he does not like. She decided she did not have the proper mindset to contract a marriage and filed for an annulment which was declined four times. She then considered becoming Orthodox simply to get a divorce in a apostolic church in good conscience despite never having seen a Divine Liturgy, read page of the Philokalia, or even knowing anything more than that they are generally like us except permitting some manner of ecclesiastically sanctioned divorce.
I have two protestant coworkers who are both on their second marriage and in both cases it looks to be built for the long haul. What of their first unions? They had something called "Biblical divorce". Divorce according to the Old Testament and the Mosaic Law? No. They both read Our Lord's words to the Samaritan woman that "But I say to you, that whosoever shall put away his wife, excepting for the cause of fornication, maketh her to commit adultery: and he that shall marry her that is put away, committeth adultery" as really meaning "If one of you cheats, then you can part." The word porneia, often translated into other languages as "adultery" or "fornication", makes little sense even in its original context. In vernacular it could be read as a permission for divorce, but in the original it is hard to see what it might mean without the lens of Tradition. Porneia, from which we derive "pornography", can mean adultery, fornication, pre-marital sex, sodomy, and, most commonly at the time, prostitution. Greek speakers went centuries reading this passage before instituting ecclesiastical divorce, and when they did so they grounded it on their understanding of ekonomia rather than this passage.
Despite people's good intentions and outward love of God, understanding of God and His commandments often becomes fungible to one's own desires. Saint Paul admonished the Corinthians that those loosed from a wife ought not seek one so as not to be solicitous to terrestrial thing things. Marriage confirms one's place in the natural order and while certainly not disconnecting one from the supernatural order, it does mean one has to balance the world of man and the City of God. The world of man is where we spend most of our time, even the most devout of us, and it rarely preaches in our faces. It whispers in our ears and accustoms us to its wants. In this world, the Word of God is bent to us rather when we should be lifted to it.
Having failed in marriage need not mean one's eternal damnation or even consignment to temporal unhappiness if one heeds the vows of marriage and seeks not another union. Those who fall can find consolation in penance, joy in friendship, and stability in the Church. Following God's commandments need not be miserable, it just may not mean always having what one wants. Is that not the message of the Cross and Resurrection?