In Parashat Eikev, the Torah says “VeAchalta VeSavata UVeirachta,” “You shall eat and be full and then bless Hashem” (Devarim 8:10). This Pasuk refers to Birchat HaMazon.
The Mishnah (Berachot 45a) presents a Machloket between the Tana Kama and Rabi Yehuda as to how much one must eat in order to be Chayav in the Mitzvah of Zimun, the introductory summons to Birchat HaMazon. The Tana Kama believes it is a KeZayit, an olive’s worth, and Rabi Yehuda says it is a KeBeitza, an egg’s worth. These criteria are extended to Birchat HaMazon.
The Gemara (Berachot 20b) in Rav Avira’s name cites a story of an interaction between the Malachim (angels) and Hashem. The Malachim say, “You said in the Torah that you don’t have favorites, but do you not favor Bnei Yisrael?” Hashem responds to them, “How could I not favor Bnei Yisrael? I said in the Torah to eat and be full and then bless Me, but they are so strict that they recite Birchat HaMazon even when they only eat a KeZayit or KeBeitza.”
The obvious question that arises is why Hashem is so impressed with a simple Chumra (stringency) that we take upon ourselves, to the point where he will go against what he said that he doesn’t have favorites? Furthermore, there are many Chumrot that we keep, so why does Hashem care so much about this specific one?
Rav Yosef Dov Soloveitchik suggests that it is based on the context in which this Pasuk is found. It is found in the context of Hashem telling us he will give us Eretz Yisrael and how great it will be, including an abundance of food. When living through these conditions, blessing Hashem isn’t so hard, as it is natural for a human to thank the One who gave him the food. Therefore, saying Birchat HaMazon is easy. But if times are tough and the entire meal that you will eat is one small KeZayit, then blessing Hashem is very hard. When Bnei Yisrael say Birchat HaMazon under these conditions, it is so unbelievable that Hashem will go against what He said and will favor the Jewish nation.