The mourning is a preparation for Redemption, and from the destruction of the past the future is built. Through teshuvah the sin itself becomes a vehicle which encourages a person to come close to G-d and serve Him. This is the simple meaning of the saying of the sages that where those who have done teshuvah stand, perfect tzadikim cannot stand. And Chazal have used a similar logic when they said that anyone who mourns over Yerushalayim merits to see her consolation. In other words, the pain of the destruction is one of the building blocks of the future construction and redemption.
Most normal people feel frightened when they are confronted by a dead body or by death in gereral.
A person is so frightened by the inevitibility of his own death that he normally manages to exclude it from his conciousness altogether. A person is so strongly attached to the material plane of existence that he identifies himself entirely with life in this world. On the other hand, the greater a person is, the more is he aware that his true identity is his soul, which is eternal and indestructible. Therefore, at least one day a year, the ninth of Av, is set aside and dedicated to mourning. On this day even study is prohibited in order that there shall not be any distractions to the contemplation and awareness that material existence is transitory and vain.