Sorrow of some kind either mingles itself with outward joy, or follows hard upon it.

Even in laughter the heart is sorrowful - Many a time is a smile forced upon the face, when the heart is in deep distress. And it is a hard task to put on the face of mirth, when a man has a heavy heart.

Even in laughter the heart is sorrowful; (h) and the end of that mirth [is] heaviness.
(h) He shows the allurement to sin, that it seems sweet, but the end of it is destruction.

Even in laughter the heart is sorrowful,.... As Belshazzar's was in the midst of his feast and jollity, when he saw the writing on the wall; so sin may stare a man in the face, and guilt load his conscience and fill him with sorrow, amidst his merriment; a man may put on a merry countenance, and feign a laugh, when his heart is very sorrowful; and oftentimes this sorrow comes by sinful laughter, by mocking at sin and jesting at religion;
and the end of that mirth is heaviness: sometimes in this life a sinner mourns at last, and mourns for his wicked mirth, or that he has made himself so merry with religious persons and things, and oftentimes when it is too late; so the end of that mirth the fool in the Gospel promised himself was heaviness, when his soul was required of him; this was the case of the rich man who had his good things here, and his evil things hereafter.

The preceding sentiment illustrated by the disappointments of a wicked or untimely joy.

13 Even in the midst of laughter the heart experiences sadness;
And to it, joy, the end is sorrow.
Every human heart carries the feeling of disquiet and of separation from its true home, and of the nothingness, the transitoriness of all that is earthly; and in addition to this, there is many a secret sorrow in every one which grows out of his own corporeal and spiritual life, and from his relation to other men; and this sorrow, which is from infancy onward the lot of the human heart, and which more and more depends and diversifies itself in the course of life, makes itself perceptible even in the midst of laughter, in spite of the mirth and merriment, without being able to be suppressed or expelled from the soul, returning always the more intensely, the more violently we may have for a time kept it under and sunk it in unconsciousness. Euchel cites here the words of the poet, according to which 13a is literally true:
"No, man is not made for joy;
Why weep his eyes when in heart he laughs?"
(Note: "Nein, der Mensch ist zur Freude nicht gemacht, Darum weint sein Aug' wenn er herzlich lacht.")
From the fact that sorrow is the fundamental condition of humanity, and forms the background of laughter, it follows, 13b, that in general it is not good for man to give himself up to joy, viz., sensual (worldly), for to it, joy, the end (the issue) is sorrow. That is true also of the final end, which according to that saying, μακάριοι οἱ κλαίοντες νῦν ὅτι γελάσετε, changes laughter into weeping, and weeping into laughter. The correction אחרית השּׂמחה (Hitzig) presses upon the Mishle style an article in such cases rejected, and removes a form of expression of the Hebr. syntaxis ornata, which here, as at Isaiah 17:6, is easily obviated, but which is warranted by a multitude of other examples, vid., at Proverbs 13:4 (also Proverbs 5:22), and cf. Philippi's Status Const. p. 14f., who regards the second word, as here שׂמהה, after the Arab., as accus. But in cases like שׂנאי שׁקר, although not in cases such as Ezra 2:62, the accus. rendering is tenable, and the Arab. does not at all demand it.
(Note: Regarding the supplying (ibdâl) of a foregoing genitive or accus. pronoun of the third person by a definite or indefinite following, in the same case as the substantive, Samachschar speaks in the Mufassal, p. 94ff., where, as examples, are found: raeituhu Zeidan, I have seen him, the Zeid; marartu bihi Zeidin, I have gone over with him, the Zeid; saraftu wugûhahâ awwalihâ, in the flight I smote the heads of the same, their front rank. Vid., regarding this anticipation of the definite idea by an indefinite, with explanations of it, Fleischer's Makkar, Additions et Corrections, p. xl. col. 2, and Dieterici's Mutanabbi, p. 341, l. 13.)
In the old Hebr. this solutio of the st. constr. belongs to the elegances of the language; it is the precursor of the vulgar post-bibl. אחרייהּ שׂל־שׂמחה. That the Hebr. may also retain a gen. where more or fewer parts of a sentence intervene between it and its governing word, is shown by such examples as Isaiah 48:9; Isaiah 49:7; Isaiah 61:7.
(Note: These examples moreover do not exceed that which is possible in the Arab., vid., regarding this omission of the mudâf, where this is supplied from the preceding before a genitive, Samachschar's Mufassal, p. 34, l. 8-13. Perhaps לחמך, Obadiah 1:7, of thy bread = the (men) of thy bread, is an example of the same thing.)

In laughter - The outward signs of joy are often mixed with real sorrow.