He explained this through a story concerning the holy Rav Moshe Leib of Sassov zt”l. Once a very wealthy man came before him, presenting a pidyon nefesh (petition for the soul) and requesting salvation, for he had fallen into a deep and grave melancholy. The greatest physicians could find no cure, nor even identify its cause. The man poured out his anguish before the Rebbe: despite his wealth and his fine, successful children, he could derive no pleasure from them, and felt no joy in life.
The holy Rebbe of Sassov said to him: The pasuk says, A psalm of David when he fled from his son Avshalom. How can it say a psalm — a term of joy — at such a time of bitter distress?
Rather, continued the Rebbe, I have received from my master and teacher, the great Maggid of Mezeritch zt”l, that whoever faces sorrow, pain, or grief must strengthen himself in joy and gladness — and by this merit he will be delivered from all troubles, as it is said (Yeshayahu 55:12), מְחָה תֵצֵאו ִ ׂ ִי בְש ּ כ - For you shall go out with joy — meaning, through joy you shall depart from all afflictions.
Therefore, the pasuk specifically says, A psalm of David — an expression of joy — to teach that precisely in that terrible time, when he was compelled to flee from his son who had become one of his greatest enemies, he took up the harp and lyre and sang to himself with joy, and thus was lifted from his sorrow.
The holy Rebbe then exhorted the man earnestly to abandon all sadness and to exert every effort in the world to remain constantly joyful. Indeed, the man obeyed the Rebbe’s counsel; and when he immersed himself in joy, in song and melody, with beautiful musical instruments, he was freed entirely from his malady and soon returned to complete health, in goodness and pleasantness.
