The Pulpit Commentary
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 (Gen_13:8; Exo_2:11; Act_7:26, etc.).

The Pulpit Commentary
It is like the precious ointment upon the head. The anointing oil of the sanctuary was an ointment composed of many "precious" ingredients, as myrrh, cinnamon, sweet calamus, and cassia, besides oil olive, which was its basis (Exo_30:23, Exo_30:24). Not only Aaron (Le 8:12), but all later high priests, were anointed with it (Exo_30:30). That ran down upon the beard, even Aaron's beard. This would be the natural result of a copious "pouring" of the oil upon the top of the head. Though not mentioned historically in Leviticus, it presents itself to the eye of the poet, on whose mental vision the whole scene rises. That went down to the skirts of his garments. Streamed even to the lower fringe of his long vesture (Kay). The high priest at his consecration was a type and symbol of unity. He bore on his breastplate the names of the twelve tribes, so that the holy oil, typical of the grace of God, when it was poured upon him, flowed down on all the tribes, diffusing everywhere an odor of fragrance.

The Pulpit Commentary
As the dew of Hermon, and as the dew that descended upon the mountains of Zion. The interpolation of the words, "and as the dew," is quite unwarrantable, and spoils the sense. It substitutes duality for unity, and destroys the parity of the two illustrations. Translate, "As the dew of Hermon, that cometh down upon the mountains of Zion." The psalmist sees the moisture which fertilizes the Holy Land, and makes it the fertile land that it is, all given forth from Hermon, the one great mountain at its head. As Dr. Kay well observes, "Physically, Hermon was to Canaan what Aaron was ceremonially to Israel—its head and crown, from which the fertilizing stores of heaven descended over the land. For not only does the one great river of Palestine, the Jordan, issue from the roots of Hermon, but the giant mountain is constantly gathering and sending off clouds, which float down even to Southern Zion." For there (i.e. in Zion) the Lord commanded the blessing, even life forevermore. The reference is to Le Psa_25:21, and perhaps to Deu_28:8.