By understanding that there is no life that always gets everything they want. So when we get something we do NOT want, and there is nothing we CAN do about it .. we relax and accept it.
Also, by understanding that the more we accept ourselves and are comfortable with ourselves - flaws and all - the less anyone's insults or blaming us.
Until then, we try to relax, accept, communicate, and see things from the other person's side too.
All relationships have conflicts once the "honeymoon/newness" phase has died out. We want what we want, or we get hurt or upset, and rather than work with OUR emotional patterns, we blame the other and try to make them change. They resist, and that is when the conflict happens.
Everyone just wants to be accepted as they are (although this isn't always possible), and no one feels loved if their partner says they are not acceptable as they are.
We either accept each other, or the relationship deteriorates and/or falls apart.
It takes most couples about 20 years to figure out how to work with their own emotions, so that the relationship stops having conflicts.