Even in laughter the heart is sorrowful; and the end of that mirth is heaviness.
Even in laughter the heart is sorrowful - (cf. Proverbs 14:10.) The Hebrew for "is sorrowful" х yikª'ab (H3510)] expresses grievous and penetrating sorrow (Job 2:13). The addition of "the heart" intensifies the sorrow (Gejer). While outwardly laughing, the man may have 'his own bitterness' within, which his "heart knoweth."
And the end of that mirth is heaviness. And even though there be no 'sorrows of' heart at the time, yet the end of mere earthly joy, however exuberant, is heaviness. Already the wise king was beginning to experience what he more fully states in Ecclesiastes 2:2; Ecclesiastes 7:6. Men's very pleasures turn into their opposites. Seek not worldly joys, which are neither solid nor lasting, but seek the joys of the Spirit, which are unmixed and everlasing.