R. Yitzchok Adlerstein, based on the writings of the Nesivos Sholom, discusses this in an article here.
Some key excerpts:
The Torah calls us banim, children of Hashem. Fatherly love comes in different forms. The most obvious is the love shared at close range. Separate the father from the son, and the love is now compounded by yearning, by the gnawing pain of distance. If we push further yet, we can detect another variety of love, still stronger than the others. Imagine a child deathly ill, desperately in need of surgery. The father is a surgeon, and only he can save the child. The child pierces his heart with his cries. When he places the blade of the scalpel against the tender skin of the child, the parent might as well be cutting into his own flesh. The love that the father feels at that moment is the strongest of all the contradictory and tempestuous emotions. Seeing the child in dire peril, realizing its utter helplessness, save for the intervention of the father – these produce a love beyond any other situation.
This is Tisha B’Av, and this is why it is a “holiday” like no other. The love of HKBH for Klal Yisrael is without condition and without bounds. In our moment of greatest weakness, and greatest vulnerability, the compassion of our Father is moved as on no other day. He displayed this love through the paradoxical embrace of the cheruvim, signifying the closeness between Hashem and Klal Yisrael at a time we might have expected Him to emphasize separation and distance.
[C]onsider what transpired at the time of the churban. Jews became aware that enemy soldiers were not only on Har Habayis, but had entered the Kodesh Kodoshim. Ordinarily, only the Kohen Gadol would enter, and only on Yom Kippur after elaborate preparation, and as part of a complex avodah. Yet now the enemy impudently entered and defiled it, mocking the Temple, its people and its G-d.
No greater insult to national pride could be contemplated. Their personal despair was now compounded by national disgrace and degradation. All who understood what had happened were tortured, broken souls. Those souls still longed for Hashem, wanted desperately to feel close again, even though – or because – they sensed that they had fallen into a spiritual pit.
Hashem cherishes the soul that is broken and turns to Him. He reacted to a nation of broken souls by displaying the cheruvim locked in embrace, as if at time of great closeness. And indeed it was. Klal Yisrael’s yearning for Him when He appeared distant led to His reasserting His commitment to them. This is the stuff a moed is made of – a time of special encounter and discovery. Tisha B’Av was indeed a holiday.