This is the meaning of what the verse says, “And -specifically- the humble will increase in their joy in Hashem.”
In Tanya, (Chapter 34) the Alter Rebbe concludes a lengthy explanation on how to serve Hashem with joy. In chapters 26-27, he explains that we should be joyful in the service of Hashem at all times, since only with the energy of joy will one be able to overcome the yetzer hara and serve Hashem with the proper vigor.
In chapters 28-31, the Alter Rebbe explains how to deal with the fact that the ego and coarseness of the animal soul encompass the G-dly soul and prevent the person from experiencing the joy hidden within his soul. He explains that the person needs to refine the ego and coarseness by having a system of making a proper accounting of where he is holding in the service of Hashem, and just how far off course he is from the proper path that Hashem wants.
A person should sometimes think about how lowly and coarse the ego and desires of his animal soul are and question himself about how much he works on himself to become refined and sensitive. When he does this, then his ego will be exposed and will begin to loosen up and allow some expression of his G-dly soul to enter his consciousness.
Now, we come full circle - if the goal of this entire process is that one should experience the joy of his G-dly soul, then how can one do that if he is focused on how unhappy he should be about his spiritual situation? How can he be expected to be happy about Hashem and sad about his spiritual exile at the same time?
The answer to this is in chapter 34, where the Alter Rebbe explains that this is possible since the joy in Hashem is from the G-dly soul and the sadness in his own shortcoming is from the animal soul, and each soul is separate. Therefore, it is possible to have both feelings be expressed in his consciousness at the same time.
Not only are they not a contradiction, but actually the very brokenness of the animal soul’s ego is what allows the person to experience the joy of the G-dly soul. This, then, is the idea of, “The humble will increase in their joy in Hashem.” This means that because of the person’s humility and selflessness, he can increase in experiencing the joy of the G-dly soul.
On Shavuos 5710 (Toras Menachem vol.1 pp. 88-90) the Rebbe quoted the Frierdiker Rebbe’s maamar on this verse, וְיָסְפו עֲנָוִּים ב ַה' ש ִּ מְחָ ה"-The humble will increase in their joy in Hashem,” and explained the idea briefly: A humble person is someone who is very strong in his opinions and he does everything with a great conviction. The fact that he is humble isn’t because he thinks he is nothing. Rather, he knows his own capabilities and strengths and knows that they are very important, but since he recognizes the truth, that everything he has is only from Hashem and what he inherited from the forefathers Avraham, Yitzchok, and Yaakov, he doesn’t attribute any greatness to himself. Therefore, he doesn’t feel himself. However, the fact that he doesn’t feel himself isn’t because he puts himself down, but because he totally puts himself aside, since he recognizes the truth that his achievements and abilities are not from his own effort and accomplishment. This type of selflessness and humility, which comes from recognizing the truth, is what allows him to experience the joy of the G-dly soul.
The Rebbe concludes by teaching that the same idea applies to a chasid and Rebbe: A chasid needs to know that the Rebbe has given him power and capability to fulfill his shlichus. As long as he knows that it’s not his power and accomplishments, but that he is just using what was given to him, then he will have the strength to accomplish whatever the Rebbe wants from him, regardless of any opposition from his own animal soul or someone else’s animal soul, since he is just going with the Rebbe’s power.
