“Love your fellow man as yourself, I am the Almighty” (Leviticus 19:18).  The Talmud (Shabbat 31a) relates that a non-Jew came to Hillel and said to him, “Convert me on condition that you will teach me the entire Torah while I stand on one foot.”  Hillel accepted the condition and told him, “What you dislike, do not do to your friend. This is the entire Torah.” Since Hillel was referring to the commandment of love your neighbor, why didn’t he just mention the words in this verse?  

Rabbi Yeruchem Levoitz explained that this is to teach us an important principle.  From the words “love your fellow man” one might think that as long as one feels the emotion of love towards others, one fulfills the commandment.  But the truth is that just feeling love alone is not sufficient. Rather this love must motivate us to do positive things for others and to refrain from any actions or words that could cause someone any pain or suffering.  The Torah definitely requires us to feel deep love for others in our hearts. But even more than that, our behavior towards others must manifest this love. Therefore, Hillel explained to this man that a basic Torah principle is that the same commandment which requires us to have a profoundly positive feeling for others, also requires us to behave in an elevated manner in our daily encounters with them (Daat Torah: Vayikra, p. 175).  Constantly be on the lookout for acts of kindness you can do for other people.  When you look for them, you will constantly find things to do and words to say. Similarly, be very careful not to do anything that will harm others or cause them any pain or suffering.  This is a lifelong challenge, and it is a cornerstone of the Torah.