In a letter from the “Alter Rebbe,” Rabbi Shneur Zalman of Liadi, he writes (Iggeres HaKodesh, 22): “Therefore, my beloved and dear ones: I beg of you to make an effort with all the heart and soul to drive into the heart the love for one’s fellow-man.”
The third Lubavitcher Rebbe, Rabbi Menachem Mendel of Lubavitch, commented on this, saying: “whoever drives the love of their fellow into their hearts, becomes beloved and dear, to the Alter Rebbe.”
(Sefer HaSichos, 5704, Pg. 22)