We have begun the month of Av. It is painful month because tradition holds that major calamities occurred on the Ninth of Av, the saddest day on the Jewish calendar.
The destruction of the Second Temple pains us the most. Why? Because we did it to ourselves. Yes, the Romans did the actual destroying, but our tradition is clear; our sinat chinam, our baseless hatred toward each other, really caused the Temple to fall. The Romans were merely G-d’s agents.