On Tishah B’Av, remembering is the rule. Tragedies happened, and we have to remember them. Not just the destruction of the Temple, indeed both Temples, but the long procession of calamities that accompanied all the ages.
Every time of year has its historical memories; every day could well have been appointed as a national Yahrzeit; but Tishah B’Av brings all the catastrophes together.
Maimonides says in his Hil’chot Evel (13:11-12) that one who does not remember and mourn for the calamities is a cruel character. And yet one can overdo the mourning. After a bereavement there are stages in mourning, and by the end of the first year the mourner must pick him- or herself up and get back into life.
The real tragedy is when there is no-one to remember. I often wandered through the Jewish part of Rookwood Cemetery when I was a rabbi in Sydney, and I would sadly note how many people had no living relative to say Kaddish and keep the Yahrzeit.
This emphasises the value of Tishah B’Av, as the occasion for the whole Jewish people to remember and mourn for the martyrs of the generations. The martyrs can never be forgotten so long as our people remembers.