The two Batei Mikdash were destroyed because of aveiros, but the aveiros of these two eras weren't the same. The primary aveiros of the first Beis HaMikdash were avodah zarah, giluy arayos, and shefichas damim. The second Beis HaMikdash was destroyed because of sinas chinam.
There were other differences between the aveiros of these two eras. The Gemara (Yoma 9) states that one of the differences is that "the first Beis HaMikdash, their aveiros were revealed, and therefore, the date when the galus would end was revealed to them. In the second Beis HaMikdash, their sins weren't revealed, and therefore, the end of the galus was not revealed."
Rashi explains that by the first Beis HaMikdash, people were transparent; they didn't conceal their true selves. Everyone knew who was a tzaddik and who was a rasha. They were "revealed," and therefore, it was revealed to them that after seventy years, they would return to Eretz Yisrael (see Yirmiyahu 29:10). But by the second Beis HaMikdash, there were resha'im who pretended to be tzaddikim. No one knew who was a true tzaddik and who wasn't. They were concealed. Therefore, the end of the galus was hidden from them. They weren't told when Moshiach would come.
Rebbe Yohonoson Eibshitz zy'a (Yaaras Dvash 7) explains the Gemara in another way. He explains the Gemara, that "during the first Beis HaMikdash, their sins were revealed," that this means the navi told them the reason for the exile. The pesukim in navi reveal the reason they were being punished, that it was for avodah zarah, giluy arayos, and shefichus damim. They knew what their sins were, so they knew how to improve. Therefore, it didn't take them long to improve their ways. Seventy years later, they returned to Eretz Yisrael.
But during the second Beis HaMikdash, their sins weren't revealed. There were no nevi'im during that era to tell them the cause of their punishment. Although the chachamim told them that the destruction was caused by sinas chinam, it was hard for the people to accept it because sinas chinam doesn't seem to be a severe aveirah. They didn't know the reason for their punishment, so it was harder for them to do teshuvah.
It is still hard for us to recognize the severity of sinas chinam, which is the reason nearly two thousand years have passed, and we haven’t yet been redeemed.
The Importance of Avoiding Sinas Chinam
The fifth of Av is the yahrtzeit of the Arizal, and he very much encouraged ahavas Yisrael. Before the tefillah, the Arizal said that one should accept on himself the mitzvah of כמוך לרעך ואהבת, to love his fellow man as himself.
Towards the end of the Arizal's short life, the Arizal created a hesger, an exclusive compound in Tzfas for his ten primary students, where he taught them the secrets of the Torah. He warned them to avoid machlokes.
But one Friday, two women got into a fight about some matter, and then their husbands got involved. Their shouts were heard from the distance.
Before Shabbos, the Arizal with his students walked out of Tzfas to be mekabel Shabbos in the fields (as this was their weekly custom). The Arizal was usually extremely happy at this time, but this week, he appeared sad. Reb Chaim Vital zt'l asked him about this, and the Arizal replied, "I heard the samach mem (Satan) say, גם אתם גם תספו מלככם, 'Also you and your king will die' (Shmuel 12:25), and I understand this to mean that I am going to die together with some of my students. And it's all because of the machlokes that occurred today. As long as there was peace among you, the Satan couldn't enter our secluded compound."
Less than a week later, on the fifth of Av, the Arizal and five of his students were niftar.
The Dangers of Lashon Hara
The Chofetz Chaim explains that sinas chinam, alone, wouldn't destroy the Beis HaMikdash. When the Gemara says the Beis HaMikdash was destroyed because of sinas chinam it refers to the severe aveirah of lashon hara (a byproduct of sinas chinam). It was lashon hara that destroyed the Beis HaMikdash. Chazal (Eiruchin 15:) say that lashon hara is just as severe as the three cardinal sins, דמים ושפיכות ,עריות גלוי ,זרה עבודה, idol worship, adultery, and murder.
The Kli Yakar (Shemos 3:2) writes, "The primary reason for the galus is their hatred and jealousy. In these regards, the Jewish nation is worse than all other nations. They speak lashon hara..."
