The Talmud (Gittin 56) tells of a man wanted to throw a party for all his friends, so he drew up a guest list and instructed his servant to send out the invitations. One of the men on the guest list was named "Kamtza," but the servant made a mistake and invited "Bar Kamtza" instead. Oops -- Bar Kamtza was actually a sworn enemy of the host!
When Bar Kamtza received his invitation, he was very grateful to think that the host had finally made amends. But when Bar Kamtza showed up at the party, the host took one look and told his servant to immediately eject Bar Kamtza from the premises.
When asked to leave, Bar Kamtza said: "I understand the mistake. But it's embarrassing for me to leave the party. I'll gladly pay the cost of my meal if you'll allow me to stay."
The host would hear nothing of this, and reiterated his demand to have Bar Kamtza removed.
Bar Kamtza appealed again: "I'd even be willing to pay half the cost of the entire party, if only I'd be allowed to stay."
Again the request was denied. At which point, the distraught Bar Kamtza pleaded: "I'll pay for the entire party! Just please don't embarrass me in this way!"
The host, however, stuck to his guns and threw Bar Kamtza out. The rabbis had observed this exchange did not protest, and Bar Kamtza took this to mean that they approved of the host's behavior.
The Talmud reports that Bar Kamtza was so hurt and upset, that he went straight to the Roman authorities and gave slanderous reports of disloyal behavior among the Jews. This fueled the Romans' anger, and they proceeded to attack and destroy the Holy Temple.
Making the Repair
We might think the Second Temple was destroyed because of a combination of complex reasons, a series of events out of our control. Not so. It is simply due to the sin of unwarranted hatred between Jews.
When we desire the redemption of our people, the key is to focus on fixing what we have ruined. The way to repair this tragedy is through the opposite of its cause: Everyone must make a maximum effort to love every member of the Jewish people. (see Chafetz Chaim: Maamar Ahavas Yisroel, Chapter 5)
This is what we are lacking today, and this is what mourn on Tisha B'Av. Every time you get into a fight with someone and you don't work it out, think long and hard that you are personally responsible for holding back the redemption. And every time you love a Jew unconditionally, we bring it another step closer.