Charles H. Spurgeon on Psalms 134:1

Behold. The Psalm begins with the demonstrative adverb Behold setting the matter of their duty before their eyes, for they were to be stimulated to devotion by looking constantly to the Temple. We are to notice the Psalmist's design in urging the duty of praise so earnestly upon them. Many of the Levites, through the tendency which there is in all men to abuse ceremonies, considered that nothing more was necessary than standing idly in the Temple, and thus overlooked the principal part of their duty. The Psalmist would show that merely to keep nightly watch over the Temple, kindle the lamps, and superintend the sacrifices, was of no importance, unless they served God spiritually, and referred all outward ceremonies to that which must be considered the main sacrifice, —the celebration of God's praises. You may think it a very laborious service, as if he had said, to stand at watch in the Temple, while others sleep in their own houses; but the worship which God requires is something more excellent than this, and demands of you to sing his praises before all the people. —John Calvin.