Note that Paul says this is his own advice, not necessarily from The Lord.
But I have known people who thought that they should follow his advice when it was obvious they were meant for marriage.
By the time they figured that out they had painted themselves into a corner.
Some people should not be married.
Some have been so damaged that they need to pick out their partner very carefully. Ruining someone else's life is not loving.
But most of us are meant to be married.
Selecting someone who is kind is a good start.
Being determined that you will be kind and supportive back even if it kills you is another.
I have seen any number of marriages fail and others that just limp along.
But when you have two people who decide that they are going to make it their goal to be kind to one another for the rest of their lives, that is the marriage that is happy.