Musar for Taanit 41:13

And thus if a man has any kind of troubles he must rejoice even so. And thus said the Sages : "Beloved are troubles" (Berakoth 5b). And the Sages said also : "He who rejoices in his pain brings salvation to the world" (Ta'anith 8a). And a man should accustom his mouth to say : "This too is for good" (Ta'anith 21a), or "All that the merciful God has done He has done for good" (Berachot 60b). For there are many apparent evils whose end is good, and thus did our Rabbis teach and interpret this portion of Scripture, "I will give thanks unto thee, O Lord; for thou wast angry with me, thine anger is turned away, and thou comfortest me" (Is. 12:1). They explained it with a parable of two men who walked, intending to board a boat. One of them had a thorn stuck into his foot so that he could not board the boat and when his companion boarded the boat the man bruised by the thorn began to curse his "evil fortune". After a time he heard that the boat had sunk and all the people in it had died. Then did he begin to praise the Creator, may He be Blessed, for he realized that the incident with the thorn had saved his life (Niddah 31a). Therefore, should a man rejoice with troubles and with other injuries that may befall him, for he does not know what good will be derived from them in the future. And thus did Nahum the man of Gamzu conduct himself (Ta'anith 21a).