At present, joy is incomplete, but in the future our joy will be absolute, not mitigated by suffering or worry.

Rabbi Yohanan said in the name of Rabbi Shimon ben Yohai: It is prohibited for a person to fill his mouth with mirth in this world, as it is stated: “Then our mouths will be filled with laughter, and our tongues with song” (Psalms 126:2). When will that be? At a time when “the nations will say: The Lord has done great things for them” (Psalms 126:2). The Sages said about Reish Lakish that after he heard this statement from his teacher, Rabbi Yohanan, he never again filled his mouth with laughter in this world.

“And you shall be completely joyous” (Deuteronomy 16:15). What is the meaning of “completely joyous”? You find that although a person can be happy in this world, his joy is never complete. How so? In this world, when children are born to a person, he worries over them whether or not they will survive, and therefore, he is always somewhat concerned. By contrast, in the messianic era, the Holy One, blessed be He, will eliminate death, as it is stated: “He will eliminate death forever” (Isaiah 25:8), and then the joy will be complete, as it is stated: “Then our mouths will be filled with laughter” (Psalms 126:2).

Alternatively: “And you shall be completely joyous.” A person is joyous in this world. A festival arrives and he acquires meat for himself, which he cooks in his home to rejoice on the festival. When he comes to eat it, he gives a portion to each of his children, and one of them inevitably complains: My brother’s portion is bigger than mine, and consequently, he is upset even in the midst of his joy. By contrast, in the messianic era, the pots will themselves cook [whatever one needs], and one will watch and rejoice over them. This is the meaning of what is written: “Lord, You will establish [tishpot] peace for us” (Isaiah 26:12). At that moment, one’s joy will be complete.

“And I will transform their mourning into gladness” (Jeremiah 31:12). For in this world, joy is ultimately transformed into mourning. How so? The seven days of mourning for the dead correspond to the seven days of a wedding feast. By contrast, in the World to Come, God says, I will restore the mourning into joy, as it is stated: “And I will transform their mourning into gladness, and I will console them and cheer them from their sorrow” (Jeremiah 31:12). (Berakhot 31a; Pesikta deRav Kahana, Parasha Aĥeret; Yalkut Shimoni 2:314)