Q - Why does cohabitation end up so often with the failure of the relationship?
A - Thanks for the question. Simply put, it is a natural result of "playing marriage" without the commitment. When any relationship has no real commitment before intimacy (on any level - emotional, mental, physical, etc.) goes too far, then the persons in the relationship, no matter their good intentions, are playing Russian Roulette with the relationship. This is because couples that cohabitate are almost twice as likely to divorce after getting married as couples who do not cohabitate are. I know of no couple that wants to divorce when they get married - so why cohabitate and risk it?
The analogy that is most commonly used is that cohabitation is like taking a car on a "test drive". The problem is that when we use people as objects, it is the worst thing we can do in a relationship. Pope John Paul II said the opposite of true love is use. This is because we make them less than human when we use them. So, when we cohabitate we are using them in every respect, whether it be for sex, companionship, intimacy, good feelings, etc. - because true love is wanting the best of someone regardless of the cost to yourself.
Furthermore, if you never have sex before marriage, you will learn along with your spouse how to be compatible with one another. There is no need to "test drive" any other person. It is offensive to even think so.
Putting a relationship in such a "danger zone" is never loving. It basically is saying to the other person (or more than likely to each other) - I see you as useful to me at this time and therefore I am willing to take a risk in hurting you physically (pregnancy, disease, etc), emotionally, spiritually and relationship-wise.
This is the short reason that cohabitation leads to such statistics. Marriage should be a permanent state - for Christians it is a covenantal and sacramental bond that is irrevocable. This permanence along with faithfulness offers a safe environment for real sacrificial love to grow. When a relationship can be changed like a shirt, love can't grow to it's fulfillment.
To make yourself a true gift to someone is the point of marriage. When you cohabitate, you are only able to give a partial gift - which points the relationship down a dead-end street.
A partial commitment is no commitment.