On the one hand vox populi says Tisha Be'Av is about the destruction of the two Temples. On the other, that this destruction came about because of baseless hatred. Thus, mourning refers to buildings (let's say divine buldings), and hatred refers to anything. That's why the third temple, the one inside the human heart, is still a possibility, not a certainty. Because we mourn stones, and hate people. If we were to mourn all those hundreds of thousands of Jews who were massacred by the armies of Nebuchadnezzar, Titus, Armleder, Rindfleisch, Chmielnicky -may their names be blotted out- insetad of the temples, the third one would lay its foundations in our hearts. If we then would never hate our fellow human being, but only their deeds and things, then the third temple would be here. And it would not be a slaughtehouse, but deep love inside us.
from the article: Dreaming of the Third Temple in a conflicted Land of Israel