Charles H. Spurgeon on Psalms 133:1

Whole Psalm. This Psalm is an effusion of holy joy occasioned by the sight of the gathering of Israel as one great household at the yearly feasts...There might likewise be an allusion to the previous jealousies and alienations in the family of Israel, which seemed to be exchanged for mutual concord and affection, on David's accession to the, throne of the whole nation. —Joseph Addison Alexander.

Behold how good and how pleasant it is, etc. There are three things wherein it is very pleasant to behold the people of God joining in one.

1. When they join or are one in opinion and judgment, when they all think the same thing, and are of one mind in the truth.

2. When they join together and are one in affection, when they are all of one heart, though possibly they are not all of one mind; or, when they meet in affection, though not in opinion. When David had spoken admiringly of this goodly sight, he spoke declaratively concerning the goodness of it (Ps 133:2): "It is like the precious ointment upon the head." 'Tis so, first, for the sweetness of it; 'tis so, secondly, for the diffusiveness of it (as followeth), "that ran down even the beard, even Aaron's beard: that went down to the skirts of his garments."

3. It is a blessed thing to see them joining together in duty, either as duty is considered —First, in doing that which is good; or, when, as the apostle's word is (2Co 6:1), they are, among themselves, "workers together" in any good work: we say (to fill up the text), "workers together with God." That's a blessed sight indeed, when we join with God, and God joins with us in his work. It is also a blessed sight when all the ministers of Jesus Christ, and many as members of Jesus Christ, join in any good work, in this especially, to beseech all we have to do with "that they receive not the grace of God in vain." Secondly, in turning from evil, and putting iniquity far from them; in praying for the pardon of sin, and making their peace with God. 'Tis a good work to turn away from evil, especially when all who are concerned in it join in it...As to join in sin, and to be brethren in iniquity, is the worst of unions, indeed, a combination against God; so to join as brethren in mourning for sin and repenting of our iniquities is a blessed union, and highly pleasing to God. —Joseph Caryl.