Please, Don't Leave Me!
Of all the holidays of the month of Tishrei, it is perhaps the very last, Shemini Atzeret, which best expresses G-d's love for the Jewish people. The name itself, "Atzeret," comes from the Hebrew word "to stop" or "delay." G-d detains us, as it were, for one more day before we return to our regular lives.
The Midrash likens this to a king who holds a seven-day celebration for his sons. On the eighth day, when it comes time for them to leave, he is reluctant to see them go and asks them to remain for one more day of festivities.
A question is asked: How can one more day of celebration make the inevitable departure less painful? What is gained by pushing it off? We must therefore conclude that there is something about this special holiday, Shemini Atzeret, that actually prevents the departure from taking place at all.
This concept is reflected in the precise language of the Midrash. "Your departure is difficult," the king tells his sons, not "our departure."
This alludes to the fact that G-d never abandons the Jewish people; His love for us is constant and eternal. "Your departure is difficult," G-d tells us. G-d doesn't want us to abandon Him; He therefore requests that we celebrate one more holiday together which will serve to strengthen our bond.
The key to maintaining a close connection with G-d is achdut--unity.
When Jews are united with one another our relationship with G-d is strong. When, however, there is strife and division, it forms a wedge between the Jewish people and our Father in heaven.
The entire theme of Sukkot is Jewish unity; indeed, the mitzva of the Four Kinds represents the four types of Jews coming together to be bound into one entity. Nonetheless, after Sukkot is over and its positive effect has dissipated, the possibility still exists that the individual elements will revert to their previous separateness and dissociation.
In order to prevent this from happening, G-d asks us to remain with Him a while longer, to celebrate a holiday which will secure our unity in an everlasting manner.
On Shemini Atzeret, a single sacrifice is brought in the Holy Temple, expressing the idea of the indivisible nature of the Jewish people. Furthermore, this concept is also reflected in the way the holiday is celebrated: great scholars and simple people alike dancing with the Torah scroll, without distinction between them.
The absolute unity with which we conclude the holidays of Tishrei thus guarantees that these feelings will carry over into the rest of the year, effectively preventing that we will ever "depart" from holiness, G-d forbid.
Adapted from Likutei Sichot of the Rebbe Vol. 2