By the time a person is in a relationship serious enough to consider marriage, they have often dates many different people. Some of those relationships were ones they ended, but others might have been terminated by the person they were seeing. When they make the decision to marry, they must know they have chosen the right partner. The failure to ensure they have not settled for someone convenient is important because their entire future will be tied up with the person they marry.
It can be difficult to think about this subject, but a person who wants a good future will take the time to ensure they have made the best choice possible. If they feel they have settled for any reason, there might be a great deal of resentment in their future. Instead of a happy couple, they will eventually become bitter and angry over their own choice. It might ruin their future, but it will also hurt the person they married.