Once the Baal Shem Tov sat with his disciples for the festive meal of Rosh Chodesh, and his face was serious. The disciples knew from previous occasions things that would put their Rebbe in a more joyous mood, and they tried, but with no success.
Then, a certain village-dwelling Jew entered, named Reb Dovid. Immediately upon his entry the Baal Shem Tov was full of joy--he treated him warmly, gave him a place to sit, and also gave him a piece of his "hamotzi" (bread).
This matter caused the disciples to have questioning thoughts--the whole Holy Society were not able to rouse joy, and only a villager was able to do so?!
The Baal Shem Tov sensed their questions, and sent Reb Dovid off to do something, and when he left he said to them:
This Reb Dovid works hard, by the sweat of his brow, for every penny he earns. During the year he saves a penny at a time to buy an etrog for Sukkot, until he assembles the sum and travels to the city and buys an extremely fine one, and he has great joy from it. Since he is poor, and his wife is of bitter spirit, it angered her considerably that he does not care about the condition of his household, since he spends money on an etrog and rejoices over it. In her bitterness she broke the pitom (tip) of the etrog (rendering it possul, unfit for use).
Now, besides the money that he spent on the etrog, there were many obstacles to bringing it, water to cross, bad roads, so that it all added up to much hard effort. Yet, when he saw what his wife had done he was not angry, but said: "It appears that I am not worthy of such an etrog. Truly, does a simple Jew like me deserve such a fine etrog?!"
"From the time of the Akeidah (the 'Binding of Isaac')," concluded the Baal Shem Tov, "there was not such a test of character, and therefore I treated him so warmly."
(Sefer Hasichot 5696, p. 148)
