It is interesting to note that being that Tisha B'Av is a day of mourning, it is forbidden to study Torah on that day. The study of Torah truly gladdens the heart and does bring a person great joy.
For many, they do not totally relate to this idea and the joy of Torah. As it is with anything of value in this world, something of real value is only attained through hard work. The same is true of Torah study. One needs to persevere for years with great diligence, to unlock the specialness and beauty of Torah. When this is accomplished, one understands how learning Torah brings tremendous joy and satisfaction to an individual.
The Mashiach is supposed to be born on Tisha B'Av. The Rabbis describe Messianic times as those where the Tzadikim will wear crowns on their heads and will be absorbed in Torah study. This is the ultimate bliss of this world and the next.
One can now understand why the only Torah study allowed on Tisha B'Av is that which pertains to the sadness of the destruction of our two holy Temples. May the third Temple be built speedily in our day when Tisha B'Av will become a Yom Tov and day of celebration.