Psalm 100 is said on weekday mornings as part of Pesukei D’Zimra, the preliminary blessings and psalms. This translation is reprinted from Rabbi Aryeh Kaplan’s anthology of sayings about prayer, A Call to the Infinite,
It is written, “Serve God with gladness, come before Him with the song” (Psalms 100:2). No sadness may be shown.
What if a person feels pain and anguish? He cannot rejoice in his heart and must seek mercy from the Supreme King in the midst of his troubles. Should he desist from praying, since he cannot do so without sadness? He cannot make his heart rejoice and enter in gladness. What can such a person do?
We are taught that other gates may be opened or closed, but the gate of tears is never closed.
Tears are only the result of sadness and anguish. Those who oversee the ways of prayer break down all locks and bars, and bring in these tears. That person’s prayer is then admitted before the Holy King.