"Israel did not have any more joyous festivals as great as Tu B'Av (15th Av) and Yom Kippur."(Mishna Taanit ch.4)

The Tiferet Shmuel explains that before the Rabbinic festivals of Channukah and Purim, there were 6 Torah festivals – Pesach, Shavuot, Sukkot, Rosh Hashanah, Yom Kippur, and Tu B'Av. How and why is Tu B'Av a Torah festival?

The second Mishna is Masechta Rosh Hashana states that G-d judges the world four times a year. On Sukkot, Pesach,  and Shavuot, G-d judges the world regarding water, grain, and fruits and vegetables. On Rosh Hashana G-d judges the entire human race.

Therefore, 4 of these 6 Torah festivals are also days of judgment. Judgment can be frightening, intimidating and distract from Simchat Yom Tov. However, on Yom Kippur and Tu B'Av there is no judgment, only unlimited Divine love. Thus, Yom Kippur and Tu B'Av are celebrated with unlimited joy.

In Pri Tzaddik, Rav Tzadok Hacohen explains that Tu B'Av was in Temple times and will again be in Messianic times, the end of a 7 day festival, the first day of which is Tisha B'Av.  As Zacharia states in Ch. 8, Tisha B'Av, when Moshiach comes, will be a great festival of joy and celebration.

Rav Tzadok's words give us a fascinating insight into the reason why the Mishna in Taanit says that Tu B'Av was a great day for matchmaking (SHIDUCHIM).

The Jewish calendar is so arranged that Tisha B'Av occurs on the same day of the week as does the first day of Pesach. The Rema states in O.C. 476 that the egg and saltwater at the Pesach Seder are reminders of the mourning and tears of Tisha B'Av.
Thus, if  Tisha B'Av is connected to the first day of Pesach, then Tu B'Av is like the 7th day of Pesach, the day of the Splitting of the Reed Sea.

The Zohar states that on the first day of Pesach, G-d chose Israel as His mate, corresponding to EIRUSIN (BETROTHAL), and on the 7th day of Pesach at the Sea of Reeds was the NISUIN (MARRIAGE) of G-d and Israel.

As Rav Tzadok Hacohen stated, Tu B'Av, the 7th day after Tisha B'Av, corresponds to the 7th day of Pesach, the day G-d married Israel. In light of the Zohar's teaching, Tu B'Av became a most propitious day to choose one's marriage mate.