When this happens, the couple may realise that they may not be suitable for each other. That's when couples break up. It's fine if they are not married yet. It'll be messy if they are and especially if they have children. That's why there is courtship.
Courtship is a time where couples get to know each other and make mistakes.
That's when we take the time to try to understand each other - ourselves included. We need to take the time to know what we want and what we want in a marriage with this potential partner. By taking the time to understand our potential life partners, we can make a more accurate assessment of the marriage surviving into old age. I do know of one fellow who rushed into marriage because a fortune teller said if he didn't get married by a certain age, he wouldn't be able to get married at all. Then there's another couple I know who were dating for 9 years and then decided to break up.