This is the Whole of Torah

A certain heathen came before Hillel and said to him, 'Make me a proselyte, on condition that you teach me the whole Torah while I stand on one foot.' Hillel responded to him, 'That which is hateful to you, do not to your fellow:  that is the whole Torah, while the rest is the commentary thereof; go and learn it.'                                                                                      Babylonian Talmud Shabbat 31a

Midrash Lekaḥ Tov [Leviticus 54a] states that the source of Hillel’s teaching “That which is hateful to you, do not to your fellow” is the verse [Leviticus 19:18]: “love your neighbor as (you love) yourself.”

However, there is an apparent lack of symmetry between the verse and Hillel’s restatement thereof. The verse implies a positive approach, the imperative to love one’s fellow as oneself, while Hillel’s statement suffices with the negative, refraining from doing to one’s fellow that which he himself finds hateful.

Many of our classic commentators stress that the Torah does not demand of us that which is not humanly possible, and note that psychologically, it is not possible for one to love another as he loves himself. (Though the Bible does present the exception of Jonathan’s love for David [I Samuel 20:17].) Malbim suggests that this point explains Hillel’s phrase: in essence, the minimum requirement of the mitzva to love another as oneself is to refrain from doing that which is hateful, but Hillel does not mean to imply that one need not strive to relate positively to one’s fellow. Indeed, Malbim comments that the Torah’s choice of word “kamocha” in the verse from Leviticus, rather than “k’nafshecha” (used in connection with Jonathan’s love for David) implies that Hillel’s statement is the accurate understanding of the verse.