Our sages say that, from the beginning of the month of Adar (that is, from Wednesday, the first day of Adar) we should add to our joy.
There are several places in the Written and Oral Torah where the concept of joy appears. Starting with the Five Books of Moses, amidst a series of very harsh statements, the idea of serving God with joy emerges. This continues in the Psalms, with the phrase "Serve God with gladness," and finally extends to Jewish philosophy, where joy is described as a tool that "breaks down barriers." It is a recurring theme in Judaism in general and Hasidism in particular. But why?
As we mentioned, "joy breaks down barriers." The simplest explanation for this is that attitude can completely change a verdict. It's not the same to show up to a job interview poorly dressed, half asleep, and sad as it is to arrive in the opposite state... in the first case, it's very likely the employer will decide not to hire you.
The same applies to joy in general; it's an attitude toward life, a way of living. Life is guided by a Master Builder who places us in the right places and at the right times. We can complain and face every situation with a bad temper, or we can simply enjoy life. It's an attitude.
However, there is a deeper explanation, and it stems from the idea that joy is not just an attitude that helps us "have a better time," but a tool that helps us to improve ourselves personally, to let our true self be free to reveal itself and, from that essential level, to overcome difficulties.
An example to understand this idea is that in moments of great joy, people do things they wouldn't normally do. For instance, a serious, structured man, the father of an only child, might dance, pirouette, and laugh like never before on his son's wedding day. Because joy breaks down the barriers of what is socially "right" and "wrong.".
And the same applies to the concept of serving God. What might seem like a burden, an obligation, a serious, dry, and mechanical duty can become something a person enjoys, something that fills them with vitality and meaning, simply by doing it joyfully. Because a person breaks free from their own limitations with joy.
Every human being in general, and the Jewish people in particular, has the capacity to awaken that joy in the service of God, simply by thinking about the idea that God, the King of all Kings, the Holy One, blessed be He, chose him to live and affect the world, chose him to be a part of His Master Plan, revealing His presence in each of the everyday things of life.