Why are we fasting? It's not our fault that the Temple was destroyed. The people at that time refused to listen to the prophets who warned them to better their ways. We are still suffering the consequences.
On this, the sages explain: "Every generation for which the Temple is not rebuilt, is as though the Temple was destroyed for that generation." If so, a fast day is not really a sad day, but an opportune day. It's a day when we are empowered to fix the cause of that first destruction, so that our long exile will be ended and we will find ourselves living in messianic times—may that be very soon.
"Because of baseless hatred between Jews," says the Talmud, "was Jerusalem destroyed."
Why, asks the Lubavitcher Rebbe, does the Talmud insist that the hate was "baseless"? Were there not reasons, both ideological and pragmatic, for the divisions amongst the Jews? But no reason, explains the Rebbe, is reason enough for hate. The commonality of our fate runs so much deeper than any possible cause for animosity. All hate, then, is baseless hatred.
So if "baseless hatred" was the cause of the destruction, continues the Rebbe, its remedy is "baseless love"--our rediscovery of the intrinsic unity which overrides all reasons for discord and strife.
Show love to a fellow Jew--no matter how he or she differs from you. For if there is one redeeming virtue in being under siege, it is the opportunity to realize that we're all in this together.