Leeches are living beings that were once applied to certain places on a person’s body to rid him of tainted blood, or to quiet the “boiling” of healthy blood (i.e., lower the blood pressure). These leeches could be found only in the summer, because in the winter, they would hide from the cold.

R. Avraham the Doctor, one of the chassidim of the Alter Rebbe, once complained that the month of Elul was nearly over and he had still not prepared the leeches. The Rebbe’s son, Rabbi DovBer (who would later become famous as the Mitteler Rebbe), was a young boy at the time, and he overheard R. Avraham’s statement. He later noticed several of his father’s chassidim conversing, and several of them burst out in laughter. “Elul is almost gone and you still have not prepared the leeches. That is why you are laughing?” he called out. Having said his piece, he left the room. The chassidim recognized the greatness of the Rebbe’s son, and so they searched for meaning in his words. They were certain that he must have been repeating something he heard from his father.

Some time went by and this story reached the Alter Rebbe himself. It was then discovered that he had never said it, but rather it was the unintended statement of R. Avraham the Doctor. Said the Alter Rebbe: “This is the path of the Baal Shem Tov. We should utilize every physical thing that we see or hear in our service of G‑d. We worked very hard to achieve this quality, but for our children it happens automatically.” He concluded: “One who follows in this manner of the Baal Shem Tov and uses everything for his service of G‑d, I promise that he will draw down a ‘good spark’ from Heaven which will assist him and his children in their service of G‑d throughout the generations.”

The Previous Rebbe, Rabbi Yosef Yitzchak, who was conveying this story, concluded his account: That year, on Rosh HaShanah, the floor of the synagogue was soaked with the tears that congregants shed when contemplating the Mitteler Rebbe’s comment. And on Simchas Torah, their shoes were torn because of the great joyful dancing that resulted from the Alter Rebbe’s promise.

What does it all mean? Both of the remedies that leeches provide have a correlation in Divine service:

Blood is warmth and life. These characteristics are the embodiment of holiness. After all, impurity and evil have no life or warmth of their own. They exist only to preserve man’s freedom of choice — “for G‑d is testing you, to know whether you will follow His ways….” This is why it is possible for a person to get excited over desires that are forbidden by the Torah, and even those that are permitted but the excitement is misplaced. Obviously, a lot of hard work is required to uproot these tendencies.

Even the “boiling blood” of holiness must sometimes be quieted. One could argue: Excitement in holy matters is a good thing, and its opposite, coldness, is the root of all kinds of evil! Why should we quiet that excitement? However, a person must ensure that his excitement springs from his connection to G‑d, rather than from personal emotions.

Excitement that is based on personal feelings, even if it is directed toward holy endeavors, can easily become the cause ofundesirable results. This is possible on the level of Nadav and Avihu, who “came too close to G‑d, and they died,” or on a lower level when, for example, someone becomes very excited and involved in the fulfillment of a specific mitzvah, but as a result of his zeal, he does it in a manner not permitted by halachah. For instance, one might be so anxious to eat the meal honoring the festivals that he does so without first washing his hands, or does so immediately upon awakening before reciting the morning blessings. Similarly, one may become excited about prayer and, as a result, may pray in a synagogue that does not have the required separation between men and women, or pray using a fabricated liturgy which is not in accordance with that established by the Sages of the Great Assembly.

Sichos Kodesh, Simchas Torah, 5723
(Translation taken from Mind Over Matter)