It is written (Tehillim 79:1): “A song to Asaf: Alien nations have entered Your estate, they have defiled Your Holy Temple, they have laid Jerusalem in ruins . . .”
Should not the verse have said “A weeping to Asaf,” “A wail to Asaf,” “A lament to Asaf”? Why does it say “A song to Asaf”?
But this is analogous to a king who built a nuptial home for his son, and had it beautifully plastered, inlaid and decorated. Then this son strayed off to an evil life. So the king came to the nuptial canopy, tore down the tapestries and broke the rails. Upon which the prince’s tutor took a flute and began to play. Those who saw him, asked: “The king is overturning the nuptial canopy of his son, and you sit and sing?” Said he to them: “I am singing because the king overturned his son’s nuptial canopy, and did not vent his wrath upon his son.”
So, too, was asked of Asaf: “G‑d destroyed the Temple and Sanctuary, and you sit and sing?” Replied he: “I am singing because G‑d vent His wrath upon wood and stone, and did not vent his wrath upon Israel.”
(Midrash Rabbah on Eichah 4:15)