Some of this couples will last forever, some will realize they are not meant for each other and they can walk away without ruining their or their partner's life.
Maybe some of them will ultimately be more happy with their lives and meet other people who they really love.
The current legal form of marriage formalizes love in a contract that doesn't take into account the fact that love can disappears one day for no reason, but assumes marriage is here to stay. Or to be more accurate, it does take that into account with a very expensive trigger called "divorce". Therefore the divorce is putting a price to my lack of love, and makes the decision to quit harder. By doing so the decision of staying together, which should be governed by love, is determined by cost, effectively negating the whole purpose of a marriage.
Unless it's purpose keeping two individuals together no matter what, even if there's no love. Maybe it made sense in the medieval era, not in our century.