King David was Israel’s greatest king though he dealt with conflict, wars, and murder even within his own family. His children fought against each other, murdered each other, raped each other, and even attempted to usurp his throne. Much of that strife was brought on as a result of his decisions earlier in life. Who would know better the need for harmony than a man whose life was filled with struggle?
Psalm 133:1 Behold, how good and pleasant it is for brothers to live together in unity (or harmony in some translations).
The idea was that living together was a good thing for fraternal blessing.
Just briefly consider “harmony”. In music, harmony does not mean everyone is singing exactly the same thing. Harmony is when more than one part are on different tunes but they go together. There is a saying that I have heard in my family that if two men always agree all the time, one of them is not necessary. It’s always nice to have a little differing opinions in order to sharpen each other. However, while we do not have to all look the same or sound the same, it is good for us to be together in harmony. It is logical then that it is not good for brothers to be in strife or fighting with each other. We don’t necessarily have to be on the same note with each other, but let’s work on our harmony.