Free translation from a talk of the Rebbe, the day preceding Rosh Chodesh Sivan, 5740 (1980) (excerpt)

We must learn a lesson from everything we encounter. Furthermore, that lesson must apply to deed for as our sages declared, “Deed is most essential.” The lesson which we can derive from the encampment of the Jews “as one man with one heart” is to carry out in actual deed, the directives of the previous Rebbeim: the Baal Shem Tov, the Maggid, the Alter Rebbe and their successors, in regard to Ahavas Yisroel (the love for one’s fellow Jew). The verse “Love your fellow-man as yourself” (Vayikra 19:18) should be interpreted in a simple manner. We must show the same unlimited and unrestrained love that we have for ourselves to others. This love must be extended to every Jew, even one in a far away corner of the world, even one whom we have never seen. Even if all we know about an individual is that he is Jewish, we must realize that he is “our fellow,” and that we are obligated to love him “as ourselves.” Furthermore, we must express this love in actual deed, helping each Jew and seeing to his welfare in both spiritual and physical matters.

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Since “Deed is most essential,” thus, in addition to the good resolutions each of us will take upon himself in regard to Ahavas Yisroel, we must realize that the resolutions alone (though they are very important) will not suffice, and we must carry them out in action e.g. by receiving every Jew “with a cheerful countenance” (Pirkei Avos 1:15), speaking to him congenially, in a manner that is based on love and lets him feel that love. Also, we must do deeds that benefit each Jew.