Fr. Timothy Pfeiffer gave a great explanation of this.
He said that, without grace, all you have is the passions.
Indeed, the passion of love is very capable of bringing two people together, and can make them do great things. But the problem is, it's a crap shoot. You never know what kind of luck you're going to have.
The flipside, explains Fr. Pfeiffer, is that the same lower nature that brought the couple together in the beginning, can rip them apart later. The problem is, neither husband nor wife ever conquered themselves, subjecting the lower nature (passions) to the higher nature (intellect/reason).
We all must conquer our passions, and subdue or at least control them. That can take a lifetime.
So among the non-Catholics, or those without religion, you CAN have a couple (more or less) happily married for 50 years. But for every one of those, you have 30 divorced couples :shocked:
Most people are not insanely lucky.
When you enter into Sacramental marriage (Matrimony), you have God's grace helping you. Also, both husband and wife understand that their soul is at stake in the marriage. They MUST "make a go of it" and get along with each other, and raise their kids Catholic, or they will have a difficult salvation, if any.
They know they only get one shot at marriage, unless their partner dies. That helps to keep both couples "trying hard" as it were.