Sometimes we have to cry in order to feel sad.
It has been noted that the laws of national mourning for the Beis HaMikdash are patterned after the personal aveilus that one observes for the loss of a relative (lo aleinu). The restrictions of The Three Weeks are the same that a mourner observes during the 12 months after losing a parent: no haircuts, music, or weddings. Beginning with The Nine Days, we take on the national version of Shloshim: no laundry, cutting nails, or bathing. Lastly, the mourning of Tish’ah B’Av itself has the status of Shiv’ah: no leather shoes, Torah study, or sitting on chairs.