A striking expression of the ultimate solitude of each man's soul at all times, and not merely at the hour of death. Something there is in every sorrow, and in every joy, which no one else can share. Beyond that range it is well to remember that there is a Divine Sympathy, uniting perfect knowledge and perfect love.

The heart knoweth his own bitterness - מרת נפשו morrath naphsho, "The bitterness of its soul." Under spiritual sorrow, the heart feels, the soul feels; all the animal nature feels and suffers. But when the peace of God is spoken to the troubled soul, the joy is indescribable; the whole man partakes of it. And a stranger to these religious feelings, to the travail of the soul, and to the witness of the Spirit, does not intermeddle with them; he does not understand them: indeed they may be even foolishness to him, because they are spiritually discerned.

The heart knoweth its own (g) bitterness; and a stranger doth not intermeddle with its joy.
(g) As a man's conscience is witness to his own grief, so another cannot feel the joy and comfort which a man feels in himself.

The heart knoweth his own bitterness,.... Or "the bitterness of his soul" (l), the distress of his conscience, the anguish of his mind; the heart of man only knows the whole of it; something of it may be known to others by his looks, his words, and gestures, but not all of it; see 1-Corinthians 2:10; bitterness of soul often arises from outward troubles, pains, and diseases of body, losses, crosses, and disappointments, 1-Samuel 1:10. Sometimes it is upon spiritual accounts; but this is not the case of every heart; men may be in the gall of bitterness, and have no bitterness of soul on account of it; the sensualist and voluptuous worldling feels nothing of it, nor the hardened and hardhearted sinner; only such who are awakened and convinced by the Spirit of God; to these, as sin is a bitter thing in itself, it is so to their taste; it makes hitter work for repentance in them; it brings trembling and astonishment on them; fills them with shame and confusion of face, causes self-loathing and abhorrence, and severe reflections upon themselves; seeing sin in its own colours, they are cut to the heart and killed with it; they are pressed down with the guilt of sin, and the load of it; and, having no views of pardon, are in that distress and bitterness of soul which no tongue can express nor heart conceive but what has felt the same;
and a stranger doth not intermeddle with his joy; or "mingle himself with it" (m); he does not share in it or partake of it; this is more especially true of spiritual joy, which, as it is unspeakable to the man that possesses it, it passes the understanding of a natural man; he can form no true idea of it: spiritual joy is what a sensible sinner partakes of upon the Gospel, the joyful sound of salvation, reaching his ears and his heart, at the revelation of Christ in him and to him, as a Saviour; when an application of pardoning grace is made to his soul, and he has a view of the complete righteousness of Christ, and his interest in it, and can see all his sins expiated and stoned for by his sacrifice; when he is favoured with a sight of the fulness of grace in Christ, and of the spiritual and eternal salvation he has wrought out for him; and likewise when he is indulged with a visit from him, and enjoys communion with him; and when he has a glimpse of eternal glory, and a well grounded hope of right unto it, and meetness for it: now a stranger, one that is a stranger to God and godliness, to Christ and the way of salvation by him, to the Spirit and his work of grace upon the heart, to the Gospel and the doctrines of it, to his own heart and the plague of it, to the saints and communion with them; knows nothing at all of the above joy, nor can he interrupt it, nor take it away.
(l) "amaritudine animae suae", V. L. Pagninus, Montanus, Mercerus, Cocceius, Gejerus, Michaelis. (m) "non immiscet se", Michaelis, so Tigurine version; "non miscebit sese", Baynus; "non intermiscet se", Junius & Tremellius, Piscator.

We do not know what stings of conscience, or consuming passions, torment the prosperous sinner. Nor does the world know the peace of mind a serious Christian enjoys, even in poverty and sickness.

Each one best knows his own sorrows or joys.

Four proverbs of joy and sorrow in the present and the future:
10 The heart knoweth the trouble of its soul,
And no stranger can intermeddle with its joy.
The accentuation לב יודע seems to point out יודע as an adjective (Lwenstein: a feeling heart), after 1-Kings 3:9, or genit. (of a feeling heart); but Cod. 1294 and the Jemen Cod., and others, as well as the editions of Jablonsky and Michaelis, have לב with Rebia, so that this is by itself to be taken as the subject (cf. the accentuation Proverbs 15:5 and under at 16a). מרּת has the ר with Dagesh, and consequently the short Kametz (Michlol 63b), like שׁרּך Proverbs 3:8, cf. כּרתה, Judges 6:28, and on the contrary כרּת, Ezekiel 16:4; it is the fem. of mōr = morr, from מרר, adstringere, amarum esse. Regarding לב, in contradistinction to נפשׁ, vid., Psychol. p. 251. "All that is meant by the Hellenic and Hellenistic νοῦς, λόγος, συνείδησις, θυμός, is comprehended in καρδία, and all by which the בשׂר and נפשׁ are affected comes in לב into the light of consciousness."
The first half of the proverb is clear: the heart, and only it, i.e., the man in the centre of his individuality, knows what brings bitterness to his soul, i.e., what troubles him in the sphere of his natural life and of the nearest life-circle surrounding him. It thus treats of life experiences which are of too complex a nature to be capable of being fully represented to others, and, as we are wont to say, of so delicate a nature that we shrink from uncovering them and making them known to others, and which on this account must be kept shut up in our own hearts, because no man is so near to us, or has so fully gained our confidence, that we have the desire and the courage to pour out our hearts to him from their very depths. Yet the saying, "Every one knows where the shoe pinches him" (1-Kings 8:38), stands nearer to this proverb; here this expression receives a psychological, yet a sharper and a deeper expression, for the knowledge of that which grieves the soul is attributed to the heart, in which, as the innermost of the soul-corporeal life, it reflects itself and becomes the matter-of-fact of the reflex consciousness in which it must shut itself up, but also for the most part without external expression. If we now interpret לא־יתערב as prohibitive, then this would stand (with this exception, that in this case אל instead of לא is to be expected) in opposition, certainly not intended, to the exhortation, Romans 12:15, "Rejoice with them that do rejoice," and to the saying, "Distributed joy is doubled joy, distributed sorrow is half sorrow;" and an admonition to leave man alone with his joy, instead of urging him to distribute it, does not run parallel with 10a. Therefore we interpret the fut. as potentialis. As there is a soul-sorrow of the man whose experience is merely a matter of the heart, so there is also a soul-joy with which no other (vid., regarding זר, p. 135, and cf. here particularly Job 19:27) intermeddleth (ההערב בּ like Psalm 106:35), in which no other can intermeddle, because his experience, as e.g., of blessed spiritual affection or of benevolent feeling, is purely of a personal nature, and admits of no participation (cf. on ἔκρυψε, Matthew 13:44), and thus of no communication to others. Elster well observes: "By this thought, that the innermost feelings of a man are never fully imparted to another man, never perfectly cover themselves with the feelings of another, yea, cannot at all be fully understood by another, the worth and the significance of each separate human personality is made conspicuous, not one of which is the example of a species, but each has its own peculiarity, which no one of countless individuals possesses. At the same time the proverb has the significance, that it shows the impossibility of a perfect fellowship among men, because one never wholly understands another. Thereby it is indicated that no human fellowship can give true salvation, but only the fellowship with God, whose love and wisdom are capable of shining through the most secret sanctuary of human personality." Thus also Dchsel (but he interprets 10b admonitorily): "Each man is a little world in himself, which God only fully sees through and understands. His sorrow appertaining to his innermost life, and his joy, another is never able fully to transfer to himself. Yea, the most sorrowful of all experiences, the most inward of all joys, we possess altogether alone, without any to participate with us."

Bitterness - The inward griefs and joys of mens hearts, are not known to any but a man's self.