Hit any key on a piano and more than one string vibrates. For instance, if you hit middle c, all of the other c’s on the piano vibrate slightly because they are all tuned to c. The vibrations of one stir up the vibrations of all the others.
David points out a similar idea in Psalm 133:1. “Behold, how good and pleasant it is when brothers dwell in unity!”
The verse speaks of those who have been tuned in to God as “brothers.” When the note of dwelling together (fellowship) is played in their lives, the vibrations of faith stir one another up in a way that is good and pleasant.
Haven’t you noticed this before? When people come together with a shared passion for Christ, there is a sense of divine vibrations among them.
In contrast, when a group of people (even in a Church) come together with their own interest and agendas taking priority over Christ, there is an unsettled tension similar to a child banging on the piano with clenched fist.