This first verse expresses the goal, the hope, the prayer of all Christians. What a great thing it would be if all the people could live together harmoniously! What things we could accomplish! What great pleasure we would have! How attractive that would be.
This verse certainly expresses the joy that results in brethren being united, when they have unanimity, when they are "at one." An irony of this "psalm of unity," however, is that the word "unity" does not literally appear in it. The literal translation of the last phrase is "when brethren dwell also together." The idea of unity is obviously there, but the final Hebrew word is yachad, meaning "together," "both," "joined." The phrase can be translated, then, "when brethren are joined in dwelling" or "when brethren dwell together." "In unity" is the translator's interpretation, not a direct translation.
The word "good" here is a fairly general rendering, but the psalmist's idea is "proper": "How proper it is for brethren to dwell together in unity!" "How fitting, how right it is for God's people to be one."
Pleasant has the sense of "attractive": "How attractive [charming, lovely] it is when God's people dwell as one." And since we are God's dwelling, we could say, "How wonderful it is when God's dwelling, the Temple of the living God, is one building and not scattered pieces all over the place."
In God's sight, unity or togetherness among His people is proper, and it pleases Him to no end. It has the same effect on us. Brethren who are thus joined together receive the benefits of the goodness and pleasantness unity produces. That is why we should yearn for this unity, because it is right, good, and fitting and because it is lovely, attractive, and appealing.
Richard T. Ritenbaugh