Psalms 118:1

It appears here, as often as elsewhere, that David had his heart full of the goodness of God. He loved to think of it, loved to speak of it, and was very solicitous that God might have the praise of it and others the comfort of it. The more our hearts are impressed with a sense of God's goodness the more they will be enlarged in all manner of obedience. In these verses, I. He celebrates God's mercy in general, and calls upon others to acknowledge it, from their own experience of it (Psa 118:1): O give thanks unto the Lord, for he is not only good in himself, but good to you, and his mercy endures for ever, not only in the everlasting fountain, God himself, but in the never-failing streams of that mercy, which shall run parallel with the longest line of eternity, and in the chosen vessels of mercy, who will be everlasting monuments of it. Israel, and the house of Aaron, and all that fear God, were called upon to trust in God (Psa 115:9-11); here they are called upon to confess that his mercy endures for ever, and so to encourage themselves to trust in him, Psa 118:2-4.

— Matthew Henry's Concise Commentary (public domain)