What do we learn from "Haysah Li Dim'asi Lechem"?
Rashi (from Shocher Tov): The affliction satiates man, and he does not seek to eat. It says also about Chanah "va'Tivkeh v'Lo Sochal" (Shmuel I, 1:7) - crying satiates!
Radak #1: I cry due to the enemies' words. My tears are constant, day and night, like food. There is no day that I do not cry twice.
Radak #2: Some say that he cries at the time of eating.
Malbim: The 'food' of my two daily meals, morning and night, is tears, because "they say to me the entire day 'where is your G-d?'" This food (tears) arouses more thirst for Kadosh water from the springs of salvation - Nevu'ah and Ru'ach ha'Kodesh. In addition to the Nefesh's yearning due to its nature, the yearning is increased via missing this light, and the Nefesh's longing that the light return to it. Food of the Nefesh is Torah, Korbanos and Avodah.
What do they say the entire day?
Radak: The entire Galus, the enemies say 'where is your G-d? If He is Elokim Emes, and you serve Him, He should save you and take you out of exile!
