The Baal Shem Tov gave the following reason as to why we do not recite the blessing for the new month on the Shabbos preceding the beginning of the month of Tishrei as we do on the Shabbasos that precede all other new months:

“The seventh month [of the festival calendar], which is the first month of the months of the year [of the calendar that begins with Creation, i.e., the month of Tishrei], is blessed by G‑d Himself on Shabbos Mevarchim, the last Shabbos of the month of Elul. Thus empowered, the Jewish people bless the [other] months, eleven times in the year.”1

The Baal Shem Tov goes on to explain what G‑d’s blessing entails: “The Torah states [in the beginning of the section of Nitzavim2 ], ‘You are all standing today....’ ‘Today’ refers to Rosh HaShanah, which is the Day of Judgment.... You (the Jewish people) stand fast and upright on this day; i.e., you are judged favorably. On the Shabbos preceding Rosh HaShanah, which is the last Shabbos of the month of Elul, we read that ‘You are all standing today....’ This is G‑d’s blessing on the Shabbos when we bless the ‘seventh month,’ [a month] that is satiated3 and satiates all Jews with abundant goodness the year round.”

We understand from the above that although the month of Tishrei is blessed by G‑d Himself, even this blessing is drawn down by the Jewish people through their reading of the Torah on the preceding Shabbos. The only difference between this and other months is that in the case of the other months the blessing itself comes from the Jewish people, while the blessing for Tishrei is recited by G‑d Himself — though in order for it to be drawn down below, the Jewish people must read the portion of Nitzavim.

Another example — and explanation — of how something is effected by G‑d but nevertheless needs the spiritual service of the Jewish people to bring it into actuality, is to be found with regard to Rosh HaShanah.

The Gemara relates4 that G‑d says to the Jewish people: “Recite unto Me on Rosh HaShanah... [verses of] kingship so that you will accept Me as your sovereign.” Since prior to “accepting Me as your sovereign,” G‑d is not (as it were) a King, how can Jews possibly accomplish this Kingship through their spiritual service?

For a “nation” can only affect that level of G‑dliness where the concept of “King” already exists; the Divine level that needs to be roused to cause the attribute of kingship to come to be desired totally transcends the finite endeavors of the nation.

This is why we say on Rosh HaShanah, “He (G‑d) chooses [our heritage...,”5 i.e., He chooses to desire to be King,]sincethe spiritual flow that is drawn down on Rosh HaShanah emanates from G‑d’s very Essence.6 It therefore comes about without any arousal at all on the part of created beings, but purely out of His free choice.

On the other hand, G‑d Himself says, “Recite unto Me ... [verses of] kingship so that you will accept Me as your sovereign” — His sovereignty is achieved through the recitation of these verses by the Jewish people. It thus follows that G‑d’s essential choice of becoming King comes about because of (and through) the Jewish people. Yet, this too — that it comes about in this fashion — is a result of G‑d’s free choice.

Because the blessing of the month of Tishrei and that which is accomplished on Rosh HaShanah both result from the service of Jews, an additional measure of blessing is drawn down upon them, with all of our people being inscribed and sealed for a good and sweet year.

Based on Likkutei Sichos, Vol. IX, pp. 184-192.