Rabbi Samphson Rafael Hirsch, commentary to Leviticus 19:18, in The Pentateuch: vol. 1: Genesis

"Love your neighbor's well-being as if it were your own; I am God," is the summarizing final maxim for the whole of our social behavior, in feelings, word, and deed. Hillel’s interpretation of this as: “That which is hateful to you don’t do to someone else” imposes complete equality of all as the guiding principle of all of our deeds, makes everyone take to heart the weal and woe of everybody else, changes selfishness…into consideration and love of one’s neighbor. The concept of “your neighbor” extends the ideas beyond the narrow confines of your fellow men to the idea of fellow creatures, so that in fact this sentence does contain the contents of the whole Torah, which indeed is nothing else, but the teaching of avoiding everything which is contrary and hateful to the happiness and well-being of ourselves and to that of the fellow creatures who enjoy existence down here in this world. [Translated by Isaac Levy]