Commentary on the Bible, by Adam Clarke, [1831], at sacred-texts.com
The comfort and benefit of the communion of saints, Psa 133:1-3.
There are different opinions concerning this Psalm; the most probable is, that it represents the priests and Levites returned from captivity, and united in the service of God in the sanctuary. This, the preceding, and the following, appear to make one subject. In the one hundred and thirty-second, the Lord is entreated to enter his temple, and pour out his benediction; in the one hundred and thirty-third, the beautiful order and harmony of the temple service is pointed out, and in the one hundred and thirty-fourth, all are exhorted to diligence and watchfulness in the performance of their duty. It is attributed to David by the Hebrew, the Syriac, and the Vulgate; but no name is prefixed in the Septuagint, Ethiopic, Arabic, and Anglo-Saxon.
Behold, how good and how pleasant - Unity is, according to this scripture, a good thing and a pleasant; and especially among brethren - members of the same family, of the same Christian community, and of the same nation. And why not among the great family of mankind? On the other hand, disunion is bad and hateful. The former is from heaven; the latter, from hell.
Like the precious ointment - The composition of this holy anointing oil may be seen, Exo 30:23; sweet cinnamon, sweet calamus, cassia lignea, and olive oil. The odour of this must have been very agreeable, and serves here as a metaphor to point out the exquisite excellence of brotherly love.
Ran down upon the beard - The oil was poured upon the head of Aaron so profusely as to run down upon his garments. It is customary in the east to pour out the oil on the head so profusely as to reach every limb.
As the dew of Hermon, and as the dew that descended upon the mountains of Zion - This was not Mount Zion, ציון tsiyon, in Jerusalem, but Sion, שיאן which is a part of Hermon, see Deu 4:48 : "Mount Sion, which is Hermon." On this mountain the dew is very copious. Mr. Maundrell says that "with this dew, even in dry weather, their tents were as wet as if it had rained the whole night." This seems to show the strength of the comparison.
For there - Where this unity is.
The Lord commanded the blessing - That is, an everlasting life. There he pours out his blessings, and gives a long and happy life.
For other particulars, see the commentators passim, and the following analysis.