The traditional reason given that Hashem allowed the destruction of the second Beit Hamikdash is because of Sinat Chinam, baseless hatred. It might also be translated as pointless hatred.
Tisha B'Av, more than any other commemorative day on the Jewish calendar, provides us with a deep dive into Jewish history. We mourn the destruction of two temples and the loss of sovereignty in ancient times. But in the Kinot we recite, we also mourn events like the Rhineland massacres during the first crusade in 1096, the York Massacre in 1190, the burning of the Talmud in Paris in 1242, the expulsion of Jews from Spain in 1492, and the Shoah in the 1940's. All of these events reverberate deeply through subsequent Jewish history, leaving their marks on who we are and what path Judaism followed.
We focus on Jewish history during happier commemorations as well, but not nearly as deeply. The old joke, that Jewish holidays have a narrative of "they tried to kill us, we won, let's eat" has some truth to it. The happiness and warmth that pervades most Jewish holidays keeps us from thinking too deeply about the massive sweep of Jewish history, the good AND the bad.
On Tisha B'Av, there's no such distraction. We can't minimize our history by way of zmirot or kugel. On Tisha B'Av, we face our history, the highs and the lows, the joys and the sorrows, the celebrations and the horrors, looking at them without filters.
Mourning doesn't mean wallowing with no purpose. There are positive reasons we fast and recall tragedy, and those reasons are many. We are meant to take away lessons that strengthen us as Jews and as human beings.
I'd like to suggest that one of those lessons has to do with the stated reason for the destruction of the second Beit Hamikdash, that of Sinat Chinam, pointless hatred. Despite the passage of 2,000 years, we still haven't gotten past that Sinat Chinam. We still display hatred, intolerance, contempt, and anger towards one another. Often these feelings are ostensibly l'shem shamayim, for heaven's sake. We're defending our conception of Judaism against those whose conception of Judaism differs from ours. But the expression of that disagreement, especially online, takes on forms that cannot be described in any other way except Sinat Chinam, pointless hatred. For what is the point of calling names and belittling others? It may make us feel better, or superior, but are we convincing others when we use such language and tactics?
Staring at our history in the face should give us some perspective. Whether the topic is the role of women, the state of Israel, politics, Jewish education, or any of another thousand topics, these issues pale in comparison to what has taken place in our history. And confronting the sweep of centuries in an unblinking manner, as we do on this day, should make us think about where these debates and disputes will be in 10 years, 100, years or 300 years. Will they be remembered? Will they have shaped Judaism? And even if they have, will our descendants look back at the disputes with respect or with contempt?
Let our disputes be like that of Hillel and Shammai or Abaye and Rava. Let us show consideration for one another. That doesn't mean papering over differences, but rather presenting our positions respectfully, engaging in constructive dialogue rather than destructive dialogue. On a day when we are forced to see the grand sweep of our history, let us approach our disagreements with humility and perspective, and a realization that we are in this together. Let us connect with one another with Ahavat Chinam.