Behold, how good and how pleasant it is for brethren to dwell together in unity! The unity described appears to be existent, and to present itself to the psalmist's vision. Hence the opening, "Behold!"
All may see it, and see how blessed and pleasant a thing it is. "Brethren" is used in the wide sense of descendants of a common ancestor (Genesis 13:8; Exodus 2:11; Acts 7:26, etc.).