For the Lord is good - For good is Yahweh. That is, He is not a being of mere "power;" he is not merely the Creator; but he is benevolent, and is, therefore, worthy of universal praise. In the former verses, his claim to adoration is founded on the fact that he is the "Creator," and has, as such, a right to our service; in this verse, the claim is asserted on account of his moral character:
(1) his benevolence;
(2) his mercy;
(3) his truth;
(a) the fact that he is a God of truth; and
(b) the fact that his truth endures, or that in all generations he shows himself to be faithful to his promises.
The first of these is his "benevolence:" "The Lord is good." As such, assuredly, God is worthy of praise and honor. A being of "mere" power we could not love or praise; a being whose power was united with malignity or malevolence, could only be the object of hatred and terror; but a being whose power is united with goodness or benevolence ought to he loved.
His mercy is everlasting - This is the "second" reason, drawn from his moral character, why he should be praised and adored. A being of mere "justice" may be feared and respected; but a character of "mere" justice would be to man an object of dread - and may be so anywhere. There are other attributes than the one of "justice," high and valuable as that may be, which are necessary to constitute a perfect character; and man, in order to find happiness and security, must find some other attribute in God than mere "justice," for man is a sinner, and needs pardon; he is a sufferer, and needs compassion; he is to die, and needs support and consolation. Besides, mere "justice" may drive its decisions over some of the kindest and tenderest feelings of human nature, for there are cases, under all administrations, where pardon is desirable and mercy is proper. It is, therefore, a ground of unspeakable joy for man that God is not a Being of "mere justice," but that there is mingled in his character the attribute of mercy and kindness. But for this, man could have no hope; for, as a sinner, he has no claim on God, and all his hope must be derived from God's infinite compassion. To all this as a ground of praise is to be added the fact that this mercy of God is "everlasting." Its fruits - its results - will extend to the vast eternity before us; and in all that eternity we shall never cease to enjoy the benefits of that mercy; never be suffered to fall back on the mere "justice" of God.
And his truth endureth to all generations - Margin, as in Hebrew, "to generation and generation." That is, forever. It is the same in every generation of the world. This is the third reason derived from the moral character of God for praising him; and this is a just ground of praise. We could not love and honor a God who was not true to his promises, and who did not himself love the truth; we could not honor one who was changeable and flexible - who loved one thing in one generation and a different thing in the next; who in one age was the friend of truth, and in the next the patron of falsehood. It is the just foundation for praise to God - our God - that he is essentially and always - in all worlds, and in all the generations of people - toward all in the universe - a Being of unchangeable benevolence, mercy, and truth. Such a God is worthy to be had in universal reverence; such a God is worthy of universal praise.

For the Lord is good - Goodness the perfect, eternal opposition to all badness and evil, is essential to God. Mercy and compassion are modifications of his goodness; and as his nature is eternal, so his mercy, springing from his goodness, must be everlasting. And as Truth is an essential characteristic of an infinitely intelligent and perfect nature; therefore God's truth must endure from generation to generation. Whatsoever he has promised must be fulfilled, through all the successive generations of men, as long as sun and moon shall last.
As this is a very important Psalm, and has long made a part of our public worship, I shall lay it before the reader in the oldest vernacular Versions I have hitherto met with, - the Anglo-Saxon and the Anglo-Scottish, with a literal interlineary translation of the former.
The Anglo-Saxon Hundredth Psalm
Rhyme ye the Lord all earth, serve the Lord in bliss;
Infare in sight his in blithness;
Wit ye, for that Lord he is God, he did us & not self we;
Folk his & sheep leeseway his; fare into gates his in confession, into courts is in hymns confess him.
Praise name his, for that winsom is; Lord thro' eternity mildheartedness his, & unto on kindred & kindred sothfastnes his
The reader will see that, in order to make this translation as literal as possible, I have preserved some old English words which we had from the Anglo-Saxon, and which have nearly become obsolete: e.g., Infare, "to go in;" blithness, "joy, exultation;" twit ye, "know ye;" did, the preterite of to do, "made, created," the literal translation of the Hebrew, עשה asah, he made; leeseway, "pasturage on a common;" winsom, "cheerful, merry;" mildheartedness, "tenderness of heart, compassion;" sothfastness, "steady to the sooth or truth, fast to truth." I might have noticed some various readings in Anglo-Saxon MSS.; e.g., Psalm 100:1 for idrymeth, "rhyme ye;" winsumiath, "be winsom, be joyful." And Psalm 100:5, for winsom, "cheerful;" swete, "sweet."
Anglo-Scottish Version of the Hundredth Psalm
1. Joyes to God al the erth; serves to Lord in gladnes.
2. Enters in his sight with joying.
3. Wittes for Lorde he is God; he made us and noght we;
4. Folke of hym, and schepe of his pasture; enters the gates of hym in schrift; hys Halles in ympnys; schryves to hym.
5. Loues his name, for soft is Lorde; withouten end in his mercy; and in generation and generation the sothfastnes of hym.
Thus our forefathers said and sung in heart and mouth and with their tongues made confession to salvation. There are but few words here which require explanation: Psalm 100:3, Wittes, "wot ye, know ye." Psalm 100:4, Schrift, "confession;" schryves, "confess ye." Verse 6, Loues, "praise ye, laud ye." Sothfastness, as above, steadfastness in the truth.

For the LORD [is] good; his mercy [is] (d) everlasting; and his truth [endureth] to all generations.
(d) He declares that we should never be weary in praising him, seeing his mercies toward us last forever.

For the Lord is good,.... Both in a providential way, and in a way of grace, and does good; he is the good Shepherd, that has laid down his life for the sheep; and the good Samaritan, that pours in the wine and oil of his love and grace, and his precious blood, to the healing of the wounds made by sin: while he was on earth, he went about doing good to the bodies and souls of men; and he continues to do good unto them, and therefore should be praised, served, and worshipped:
his mercy is everlasting; or "his grace" (e); there is always a sufficiency of it for his people; and his lovingkindness, which may be also here meant, is always the same; having loved his own which were in the world, he loves them to the end, John 13:1.
and his truth endureth to all generations; or his faithfulness in fulfilling his promises, and performing his engagements; he was faithful to his Father that appointed him, and to the covenant he made with him; and he is faithful to his people, to keep what they commit to his care and charge now, and to give them the crown of righteousness at the last day, which is laid up for them; and upon all these considerations, and for these reasons, ought to be praised and adored.
(e) "gratia ejus", Cocceius, Gejerus; "gratia et misericordia ejus", Michaelis.

The reason: God's eternal mercy and truth (Psalm 25:8; Psalm 89:7).