This passage is properly regarded as the Epilogue of the whole book; a kind of apology for the obscurity of many of its sayings. The passage serves therefore to make the book more intelligible and more acceptable.
Here, as in the beginning of the book Ecclesiastes 1:1-2, the Preacher speaks of himself Ecclesiastes 12:8-10 in the third person. He first repeats Ecclesiastes 12:8 the mournful, perplexing theme with which his musings began Ecclesiastes 1:2; and then states the encouraging practical conclusion Ecclesiastes 12:13-14 to which they have led him. It has been pointed out that the Epilogue assumes the identity of the Preacher with the writer of the Book of Proverbs.

This affecting and minute description of old age and death is concluded by the author with the same exclamation by which he began this book: O vanity of vanities, saith Koheleth, all is vanity. Now that man, the masterpiece of God's creation, the delegated sovereign of this lower world, is turned to dust, what is there stable or worthy of contemplation besides? All - All is Vanity!

Vanity of vanities, saith the preacher,.... The wise man, or preacher, set out in the beginning of the book with this doctrine, or proposition, which he undertook to prove; and now having proved it by an induction of particulars, instanced in the wisdom, wealth, honours, pleasures, and profit of men, and shown the vanity of them, and that the happiness of men lies not in these things, but in the knowledge and fear of God; he repeats it, and most strongly asserts it, as an undoubted truth beyond all dispute and contradiction, that all things under the sun are not only vain, but vanity itself, extremely vain, vain in the superlative degree;
all is vanity; all things in the world are vain; all creatures are subject to vanity; man in every state, and in his best estate, is altogether vanity: this the wise man might with great confidence affirm, after he had shown that not only childhood and youth are vanity, but even old age; the infirmities, sorrows, and distresses of which he had just exposed, and observed that all issue in death, the last end of man, when his body returns to the earth, and his soul to God the giver of it.

Solomon repeats his text, VANITY OF VANITIES, ALL IS VANITY. These are the words of one that could speak by dear-bought experience of the vanity of the world, which can do nothing to ease men of the burden of sin. As he considered the worth of souls, he gave good heed to what he spake and wrote; words of truth will always be acceptable words. The truths of God are as goads to such as are dull and draw back, and nails to such as are wandering and draw aside; means to establish the heart, that we may never sit loose to our duty, nor be taken from it. The Shepherd of Israel is the Giver of inspired wisdom. Teachers and guides all receive their communications from him. The title is applied in Scripture to the Lord Jesus Christ, the Son of God. The prophets sought diligently, what, or what manner of time, the Spirit of Christ in them did signify, when it testified beforehand the sufferings of Christ, and the glory that should follow. To write many books was not suited to the shortness of human life, and would be weariness to the writer, and to the reader; and then was much more so to both than it is now. All things would be vanity and vexation, except they led to this conclusion, That to fear God, and keep his commandments, is the whole of man. The fear of God includes in it all the affections of the soul towards him, which are produced by the Holy Spirit. There may be terror where there is no love, nay, where there is hatred. But this is different from the gracious fear of God, as the feelings of an affectionate child. The fear of God, is often put for the whole of true religion in the heart, and includes its practical results in the life. Let us attend to the one thing needful, and now come to him as a merciful Saviour, who will soon come as an almighty Judge, when he will bring to light the things of darkness, and manifest the counsels of all hearts. Why does God record in his word, that ALL IS VANITY, but to keep us from deceiving ourselves to our ruin? He makes our duty to be our interest. May it be graven in all our hearts. Fear God, and keep his commandments, for this is all that concerns man.

A summary of the first part.
Vanity, &c.--Resumption of the sentiment with which the book began (Ecclesiastes 1:2; 1-John 2:17).

"O vanity of vanities, saith Koheleth, all is vain." If we here look back to Ecclesiastes 12:7, that which is there said of the spirit can be no consolation. With right, Hofmann in his Schriftbeweis, I 490, says: "That it is the personal spirit of a man which returns to God; and that it returns to God without losing its consciousness, is an idea foreign to this proverb." Also, Psychol. p. 410, it is willingly conceded that the author wished here to express, first, only the fact, in itself comfortless, that the component parts of the human body return whence they came. But the comfortless averse of the proverb is yet not without a consoling reverse. For what the author, Ecclesiastes 3:21, represents as an unsettled possibility, that the spirit of a dying man does not downwards like that of a beast, but upwards, he here affirms as an actual truth.
(Note: In the Rig-Veda that which is immortal in man is called manas; the later language calls it âtman; vid., Muir in the Asiatic Journal, 1865, p. 305.)
From this, that he thus finally decides the question as an advantage to a man above a beast, it follows of necessity that the return of the spirit to God cannot be thought of as a resumption of the spirit into the essence of God (resorption or emanation), as the cessation of his independent existence, although, as also at Job 34:14; Psalm 104:29, the nearest object of the expression is directed to the ruin of the soul-corporeal life of man which directly follows the return of the spirit to God. The same conclusion arises from this, that the idea of the return of the spirit to God, in which the author at last finds rest, cannot yet stand in a subordinate place with reference to the idea of Hades, above which it raises itself; with the latter the spirit remains indestructible, although it has sunk into a silent, inactive life. And in the third place, that conclusion flows from the fact that the author is forced by the present contradiction between human experience and the righteousness of God to the postulate of a judgment finally settling these contradictions, Ecclesiastes 3:17; Ecclesiastes 11:9, cf. Ecclesiastes 12:14, whence it immediately follows that the continued existence of the spirit is thought of as a well-known truth (Psychol. p. 127). The Targ. translates, not against the spirit of the book: "the spirit will return to stand in judgment before God, who gave it to thee." In this connection of thoughts Koheleth says more than what Lucretius says (ii. 998 ss.):
Cedit item retro, de terra quod fuit ante,
In terras, et quod missum est ex aetheris oris
Id rursum caeli rellatum templa receptant.
A comforting thought lies in the words נתנהּ אשׁר. The gifts of God are on His side ἀμεταμέλητα (Romans 11:29). When He receives back that which was given, He receives it back to restore it again in another manner. Such thoughts connect themselves with the reference to God the Giv. Meanwhile the author next aims at showing the vanity of man, viz., of man as living here. Body and spirit are separated, and depart each in its own direction. Not only the world and the labours by which man is encompassed are "vain," and not only is that which man has and does and experiences "vain," but also man himself as such is vain, and thus - this is the facit - all is הבל, "vain."

Vanity - This sentence, wherewith he began this book, he here repeats in the end of it, as that which he had proved in all the foregoing discourse, and that which naturally followed from both the branches of the assertion laid down, Ecclesiastes 12:7.