The Torah rejoices with Bnei Yisrael

וענין שמחת תורה כי התורה שמחה עם בנ"י.

And the matter of Simchas Torah is that the Torah rejoices together with Bnei Yisrael.

Simchas Torah is mutual: not only do we rejoice with the Torah, the Torah itself rejoices with Bnei Yisrael.

דאיתא במד' ואהי' אצלו אמון אומן שהתורה יעצה לברוא העולם.

For the Midrash teaches on "I was beside Him as an amon" (Mishlei 8:30) — read it as "uman" (craftsman): that the Torah advised in the creation of the world.

The Torah was the "craftsman's plan" by which Hashem created the world, the blueprint and counsel for all of creation.

כמ"ש בראשית בשביל התורה שנק' ראשית ובשביל ישראל שנקראו ראשית.

As it is expounded on "Bereishis" — for the sake of the Torah, which is called "reishis," and for the sake of Yisrael, who are called "reishis."

The world was created "for the sake of reishis" — both the Torah and Bnei Yisrael, each called "reishis," are the purpose of creation.

וכל שורש הדין בר"ה וביוהכ"פ הוא לקיום העולם כפי עבודת בנ"י.

And the entire root of the judgment on Rosh Hashanah and Yom Kippur is for the upholding of the world according to the avodah of Bnei Yisrael.

The whole purpose of the High Holiday judgment is to sustain the world, and that sustaining depends on the avodah (service) of Bnei Yisrael.

ולכן כשזוכין במשפט והקב"ה מחדש עולמו התורה שמחה.

Therefore, when we are vindicated in judgment and the Holy One renews His world, the Torah rejoices.

When Bnei Yisrael emerge meritorious and Hashem renews creation, the Torah — whose purpose is thereby fulfilled — rejoices.

וכמו כן בודאי עכ"פ עיקר השמחה מכח בנ"י.

And likewise, certainly the essential joy comes from the power of Bnei Yisrael.

The main cause of this rejoicing is Bnei Yisrael themselves, through whom the Torah's purpose is realized.

שאין מי שמקיים תורה ומצות בעולם כבנ"י ולכן יש לנו לשמוח בשמחת התורה:

For there is none in the world who fulfills Torah and mitzvos like Bnei Yisrael, and therefore it is fitting for us to rejoice on Simchas Torah.

Since only Bnei Yisrael truly uphold the Torah and its mitzvos, the joy of Simchas Torah rightly belongs to them.

Summary: Simchas Torah is a shared joy in which the Torah itself rejoices with Bnei Yisrael. The Torah was the craftsman's counsel in creation, and the world was made "for the sake of reishis" — both the Torah and Bnei Yisrael. Since the High Holiday judgment exists to sustain the world through the avodah of Bnei Yisrael, when they are vindicated and Hashem renews creation, the Torah's purpose is fulfilled and it rejoices — for none uphold Torah and mitzvos like Bnei Yisrael.