Much of Torah portion Ki Teitzei relates to forbidden mixtures, including “Do not plow with an ox and a donkey together.” (Deut. 22:10). Tosafos (12-13th cent.) provides a fascinating reason for this prohibition against “mixed species.” An ox chews its cud, but a donkey does not. If an ox and a donkey are yoked together, the donkey will see the ox chewing constantly and think that the ox is getting extra food. This will cause the donkey to feel cheated and become envious of the ox. By refraining from yoking these two beasts of burden together, we avoid inflicting psychological pain on the donkey.
Rabbi Chaim Shmuelevitz (1902-1979) turns this teaching into a universal lesson. If the Torah is so concerned with the feelings of an animal, how much more should we be concerned with the feelings of our fellow humans! We should not talk about our achievements or our possessions in front of someone who might become envious. By guarding our speech in this way, we avoid leading others to violate the 10th commandment: “You shall not covet….”