Free translation from a talk of the Rebbe, Shabbat Parshat Haazinu, Shabbat Shuvah, 5745 (1984) (excerpts)

There is a lesson from all of the above. Although one has passed through all the lofty levels of repentance of the month of Elul, the days of selichos, and Rosh HaShanah, a yet higher level is added on Shabbos Shuvah, when all aspects of service are permeated with delight. Simply put, all aspects of service throughout the year, in thought, speech and deed, — i.e., every observance of Torah and mitzvos — become permeated with the concept of delight.

This lesson applies also to one’s efforts to help others, consonant to the command, ‘love your fellow as yourself.’ A person may meet a Jew who needs to repent, a repentance not just of the highest level, that of ‘the spirit shall return to the G‑d who gave it,’ but repentance of a lower level, for plain misconduct. One may be tempted to admonish such a Jew about his misdeeds. Shabbos Shuvah teaches that instructing a Jew about repentance should be done with delight — pleasantly and peacefully, drawing him near with love, a love that is ‘as strong as self-love, for ‘You shall love your fellow as yourself.’ As is known, the Baal Shem Tov would first help Jews in their material matters — a relationship of love — and only then speak with them about spiritual matters, Torah and mitzvos.

On the other hand, one must be careful not to think that because Shabbos Shuvah is so lofty, to the extent that it comes from a level unattainable by man, and that he is together with G‑d — that therefore there is no need to be concerned about the fact that others may have transgressed and need to repent. This is a mistaken attitude, for G‑d wants a dwelling place in this low, material world specifically. A Jew must therefore see to it that the world be fit for G‑d’s abode, a place about which G‑d can say, ‘I shall dwell within them.’

May it be G‑d’s will that every Jew use the powers granted to him on these auspicious days, especially the strength given on Shabbos Shuvah.

* * *

In the light of the above, it also follows that the lesson previously derived from Shabbos Shuvah — that one should help another Jew (‘love your fellow as yourself’) in a manner of delight — is more emphatically stressed this year, when Shabbos Shuvah immediately follows Rosh HaShanah. For Rosh HaShanah emphasizes the unity of Jews, as in the verse, ‘You are all standing here today before the L‑rd your G‑d,’ ‘today’ referring to Rosh HaShanah. This is an expression not just of love of fellow Jews but even more, unity between Jews. ‘You shall love your fellow as yourself’ means that although there is love between the two, they are still somewhat separate entities. Unity between Jews means they are one.

When, therefore, Shabbos Shuvah immediately follows Rosh HaShanah, there is a heavy emphasis on the fact that helping another Jew (stressed on Rosh HaShanah) should be done with delight (as derived from Shabbos Shuvah).

Through increasing in love of and unity between Jews, we merit an increase in G‑d’s blessings, as we say in our prayers, ‘Bless us our Father, all of us as one, with the light of Your countenance.’