Rav Wolbe quotes an astounding Chazal (Vayikra Raba 26, 2) that puts the importance of good middos in the proper perspective. In the days of Dovid Hamelech, even the small children were so proficient in Torah that they were able to explain each law of the Torah in ninety-eight different ways. Despite the generation's greatness in Torah, because they spoke lashon hara, when they waged war against their enemies they fell in battle. In contrast, the entire generation of King Achav worshipped idols and, nevertheless, since they refrained from speaking lashon hara they were victorious in battle.
How could it be that the tremendous amount of Torah study did not protect the people of Dovid Hamelech's generation from defeat? Additionally, why were there gossipmongers in the times of Dovid Hamelech; doesn't Torah study automatically generate good middos? However, the truth is, with regard to spiritual growth nothing is automatic. One who does not make a conscious effort to work on himself, will remain deficient in that area. It is evident from Chazal that a deficiency with regard to middos is so great, that it overrides other mitzvos and tips the scales against the Jewish people.
Another example of the severity associated with being derelict in middos is the destruction of the second Bais Hamikdosh. Chazal tell us that although those who lived at that time studied Torah, performed mitzvos and did acts of charity, since they were guilty of sinas chinam the Bais Hamikdosh was destroyed. This demonstrates the severity of bad middos to the extent that all the mitzvos that they did could not rectify their shortcomings nor prevent the calamity that such bad middos precipitate. This brings us to the realization that there is no way out, other than to make make a conscious effort to work on developing good middos. Such qualities don't simply come by themselves, and when there are shortcomings in this area, the consequences can be catastrophic chas v'shalom.
Hatred has no place among the Jewish people - especially among those who spend their days immersed in Torah study. The Mishna (Sanhedrin 3, 5) tells us, "What is an expression of hatred? When one, as a result of his hatred, does not speak to another person for three consecutive days."
When we mourn the destruction of the Bais Hamikdosh - we are mourning the fact that it does not stand today. Our Sages tell us, "In every generation that the Bais Hamikdosh has not been rebuilt, it is as if the Bais Hamikdosh has once again been destroyed." Our generation has not been cleansed from the negative trait of hatred. During the days leading up to Tisha B'av, and all the more so on Tisha B'av itself, we should make a conscious effort to purge ourselves from sinas chinam, restore harmony to those relationships that were neglected and once again talk to those people with whom we had resolved not to speak. If the amount of sinas chinam amongst Bnei Yisroel is diminished as a result of Tisha B'av - then the fast will have been worthwhile.