Psalms 133:1

Here see, I. What it is that is commended - brethren's dwelling together in unity, not only not quarrelling, and devouring one another, but delighting in each other with mutual endearments, and promoting each other's welfare with mutual services. Sometimes it is chosen, as the best expedient for preserving peace, that brethren should live asunder and at a distance from each other; that indeed may prevent enmity and strife (Gen 13:9), but the goodness and pleasantness are for brethren to dwell together and so to dwell in unity, to dwell even as one (so some read it), as having one heart, one soul, one interest. David had many sons by many wives; probably he penned this psalm for their instruction, to engage them to love another, and, if they had done this, much of the mischief that arose in his family would have been happily prevented. The tribes of Israel had long had separate interests during the government of the Judges, and it was often of bad consequence; but now that they were united under one common head he would have them sensible how much it was likely to be for their advantage, especially since now the ark was fixed, and with it the place of their rendezvous for public worship and the centre of their unity. Now let them live in love. II.

— Matthew Henry's Concise Commentary (public domain)