The Fast
Rebbe Mendel of Riminov zt'l taught that since the Torah told us to fast, this means that the fast itself nourishes the person and feeds him. Therefore, he would say, it isn't necessary to be too lenient. He also said that this fast is mesugal for arichus yamim (a long life) because it heals all limbs of his body.
Yesod v'Shoresh HaAvodah (ch.10) writes, "it is obvious that there is a great reward for this mitzvah because he has physical pain to keep this mitzvah... My brother and friend, everyone should understand rationally that he should be extremely happy with every moment that he has tzaar from the refraining from food and drink and from wearing shoes... When someone truly loves Hashem, he will desire that the fast be twice as long, and even longer, so he can keep the mitzvah of the tzaar of the fast, which is a mitzvas asei of the Torah that Hashem commanded him."
However, when a reliable rav tells him that he must eat, he must undoubtedly do so, because there is an obligation of (Devarim 4:15) לנפשתיכם מאד ונשמרתם, to guard one's life and to put oneself into danger. The Shem MiShmuel wrote the following letter to his son-in-law (printed at the end of Shem MiShmuel, Moadim): "As Yom Kippur is approaching, I must warn you that you shouldn't be overly righteous regarding this fast. Do as the doctors tell you... The same Hashem Who commanded us to fast, He also commanded us to listen to the doctors and not fast... Even if the ill person claims that he can fast, and the doctors say he can't fast, we listen to the doctors' opinion (Yoma 83a). Even if the patient is an expert, and he knows the nature of fasts and health, his opinion isn't valid for his own health, and he must follow the view of the doctors.
"The same applies to the tefillos. Chazal say that the chazan is מוציא the people who are in the fields [and can't go to the beis kneses]. Even if the chazan is on the other side of the world, and not where you live, his tefillos are motzi you, as if you davened... Therefore (Koheles 9:7), מעשיך האלקים רצה כבר כי יינך טוב בלב ושתה ,לחמיך בשמחה אכול, 'Eat your bread with joy, drink wine with a happy heart, because Hashem desires your deeds.' And the Midrash (Bamidbar Rabba 17:2) says that this pasuk is what the bas kol announced on Yom Kippur, in the days of Shlomo HaMelech, when the nation ate on Yom Kippur, to celebrate the dedication of the Beis HaMikdash."
Someone told the Chazon Ish zt'l that he has tzaar that he must eat on Yom Kippur. The Chazon Ish replied, "I never heard someone say that he has tzaar that he has to make a bris milah on Shabbos." (Maaseh Ish 4, p.163)
Rebbe Yehoshua of Belz wrote the following letter to Rebbe Mendel of Vizhnitz zt'l (5655): “To my beloved mechutan, the holy tzaddik, the pride of the Jewish nation, Rebbe Mendel shlita: I write this letter because I was shocked to hear that your chassidim are worried that you might fast on the holy day, Yom Kippur. Who would believe a holy man like you would do such a thing? Hashem, Who told us to fast on Yom Kippur, also told us to guard our lives. I remember when my father, Rebbe Shalom of Belz, was ill, we feared that he might fast on Yom Kippur. But he was righteous. Immediately after Kol Nidrei, he asked that we bring him food. Before eating, he said, “Hareinu muchan... I am prepared to do the mitzvah of my Creator to preserve my life,” and ate his food joyfully. Such joy we only witnessed when he ate matzah at the Seder and shook the lulav. Certainly, you will also be cautious with this mitzvah and follow the doctors’ orders. Especially since you are a great person, and people watch what you do. If you are strict and don’t eat, others will learn from you, which could be dangerous. “Believe me, I didn’t want to write this letter to advise you on how to act. I’m writing this letter against my will because I care for you so much. Hashem should bless your bread and water, remove your illness, and send you a complete and speedy refuah. “These are the words of your mechutan, who truly loves you and hopes and trusts to hear good news about your good health.”
Someone had to eat on Yom Kippur. He feared that this meant Hashem didn't want his service. He compared it to the Gemara (Succah 28b), which discusses when it rains on Succos. The Gemara says, "A mashal to a person who is pouring wine for his master, and his master pours it in his face." The Gemara says that the same is when it rains on Succos. Hashem is saying that He doesn't want our service. This is what occurs on Yom Kippur. Hashem comes to us, and thereby, our aveiros are removed.
The Aruch L'Ner (Binyan Tzion HaChadashos siman 25) explained to him that it isn't so. When it rains on Succos, he leaves the mitzvah of succah, and he doesn't have another mitzvah. But when one eats on Yom Kippur to preserve his health, he is performing the mitzvah of לנפשתיכם מאד ונשמרתם, to be cautious with his life. Therefore, it isn't a negative sign at all. (The Aruch L'Ner added several other differences between eating on Yom Kippur and when it rains on Succos.)
