Albert Barnes' Bible Commentary
Psalms 133 - Introduction
This psalm is entitled “A Song of Degrees of David.” It is one of the four in this collection ascribed to him, and there is no reason to doubt the correctness of the inscription. As to the occasion on which the psalm was composed, however, we have no information. Perhaps there was nothing special in the occasion which called it forth, since it may have been written at any time to set forth the beauty and the power of brotherly love. It may have been composed either for the service of the people when gathered in their annual festivals, or in view of the harmony - the beauty and order - evinced when they were thus gathered together.
The psalm is an illustration, in most beautiful language, of brotherly love, particularly in regard to its calm, and gentle, and sweet influence - like the ointment which flowed down from the head of the anointed priest, or like the gentle dew on Hermon or Zion. It is a psalm applicable alike to a church; to family; to a gathering of friends.