Psalm 135 is a call to praise the Lord. We’ve seen those psalms before but this one differs from the rest in that it gives reasons for praising Him, reasons that have specifically to do with his relationship with Israel.
“The Lord” is the covenant name of the God of Israel, the name of their God who has entered into a unique agreement with them. Whenever the writer refers simply to the greatness or the works of God in this psalm, he refers to God as simply “He.” But when he refers to God’s special relationship with Israel, it’s always “The Lord.” Notice how he alternates the use of these references (“The Lord” vss. 1-6, 13-14, 19-21, “He” vss. 7-12,15-18).
The Psalm is a reminder that we (God’s people) and the Lord, have a special relationship not present among those who are not His people.