Musar zu Tehillim 126:2

And when should a person rejoice greatly and laugh? When the Divine Presence returns to Zion, for this is a great joy, as it is written "Then will our mouths be filled with laughter and our tongues with song; then will they say among the nations, 'The Lord has done great things with these' " (Ps. 126:2).

When one can pay "lip-service" – as in the case of the substitution of prayer for the sacrifice – one's lips move normally and do not display a sign that their owner grieves over the fact that he has to content himself with such "lip-service." Nowadays, when in exile, our "lip-service" is accompanied by our lips becoming painfully distorted when we really grieve over our inability to fulfil our duty in the optimal manner. To contrast this experience with the way we shall feel after the Temple will be restored, David says in Psalms 126,2: אז ימלא שחוק פינו ולשוננו רנה, "Then our mouths shall be filled with laughter and our tongues with songs of joy."

Rabbi Yochanan said (Berachos 31a): “It is forbidden to fill one’s mouth with laughter in this world, as it is stated, ‘Then [in the future] our mouth will be fill with laughter’ (Tehillim 126:2).” The Rabbis said of Rabbi Shimon ben Lakish that from the moment he heard this from Rabbi Yochanan he never again filled his mouth with laughter.