To feel true happiness, it is perhaps necessary to go through the darkness of pain and the pit of anguish.
In order to truly know happiness, one must make a place for it in oneself, and this can best be done by great pain which thrusts all else aside.
For example, the greatest happiness of all, the pleasure of being alive, is hardly ever experienced in ordinary circumstances.
And only when life is threatened, in passing through the danger of death, does one know it fully.
In other words, only when a person realizes the full pain and terror of his life, can he make a place for God in himself.
But of course, this applies to the times in history or in personal life when a person can allow himself the luxury of experiencing sadness at fixed times, when one is not the victim or the object of suffering.
Sadness is well and good if it can be taken out and put away at will.
As the instructions in certain old prayer books directed: "Here one is to weep …. "
The fact is that life was perhaps harder in the old days, and in order to overcome the immense sadness of it, Jews had to put aside certain times for grief and weeping.
It used to be a wry joke among Jews to say to someone full of complaints against fate, "Save it for the proper occasion in the course of prayer."
–Rabbi Adin Steinsaltz
From The Long Shorter Way by Rabbi Adin Steinsaltz