Charles H. Spurgeon on Psalms 135:20

Bless the LORD, O house of Levi. These helped the priests in other things, let them aid them in this also. The house of Israel comprehends all the chosen seed; then we come down to the smaller but more central ring of the house of Aaron, and now we widen out to the whole tribe of Levi. Let reverence and adoration spread from man to man until the whole lump of humanity shall be leavened. The house of Levi had choice reasons for blessing God: read the Levite story and see. Remember that the whole of the Levites were set apart for holy service, and supported by tim tribes allotted to them; therefore they were in honour bound above all others to worship Jehovah with cheerfulness.