The following is excerpted from Rabbi Ginsburgh's new book, "Joyous All Year Round." You can order this book here.
On the three pilgrimage festivals (Pesach, Shavuot, and Sukkot), we recite the joyous prayer of praise of God, called Hallel. Chanukah is a festival of both praise and thanksgiving, and Purim is a holiday of thanksgiving alone. (The thanksgiving on Chanukah and Purim is the Al Hanisim prayer, in which we thank God for the miracles that He engendered on these holidays).
The Talmud provides several explanations for why we do not say Hallel on Purim. The inner reason is so that we can highlight the dimension of thanksgiving on Purim.
On the holidays, we experience the soul's surrounding powers, the superconscious above our ordinary consciousness. These are the levels of chayah (the living one) and yechidah (the singular one) in the soul. The Hallel prayer expresses the makif karov (closely surrounding light) of the chayah, which illuminates the soul. The root of thanksgiving is in the concealed makif rachok (distant surrounding light) of the yechidah, which is the source of Mordechai's self-sacrifice in the Purim story. The yechidah is a lofty, 'masked' place, which requires us to admit the unseen, concealed truth.
Thanksgiving is also the place in the soul to where joy extends. Kabbalah depicts this as "Understanding extends to Thanksgiving." The sefirah of understanding, whose inner dimension is joy, extends until the sefirah of hod, thanksgiving.
The holiday of thanksgiving and self-sacrifice reveals the joy that bursts forth "until one does not know the difference between 'cursed is Haman' and 'blessed is Mordechai.'"