1....We are taught in this Psalm, when we chaunt Allelujah, which means, Praise the Lord, that we should, when we hear the words, “Confess unto the Lord”, praise the Lord. The praise of God could not be expressed in fewer words than these, “For He is good.” I see not what can be more solemn than this brevity, since goodness is so peculiarly the quality of God, that the Son of God Himself when addressed by some one as “Good Master,” by one, namely, who beholding His flesh, and comprehending not the fullness of His divine nature, considered Him as man only, replied, “Why do you call Me good? There is none good but one, that is, God.” And what is this but to say, If you wish to call Me good, recognise Me as God? But since it is addressed, in revelation of things to come, to a people freed from all toil and wandering in pilgrimage, and from all admixture with the wicked, which freedom was given it through the grace of God, who not only does not evil for evil, but even returns good for evil; it is most appropriately added, “Because His mercy endures for ever.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)