When it was time to let Bnai Yisrael into Eretz Yisrael, Bnai Yisrael asked to send spies to see what the land was like. Moshe picked twelve people, one from each tribe, to explore the land. Unfortunately, ten of them gave a bad report about the land.
These spies had a choice to give either a positive report or negative report. They chose the latter, and Bnai Yisrael unfortunately believed them. The Chachamim say that Bnai Yisrael wept that night for no purpose, so Hashem gave them something to cry about. He eventually destroyed both Batei Mikdash on that day of the year.
We can learn from this episode that we should look at everything in a positive way. We should learn from the mistake of the Meraglim to give the benefit of the doubt and look at things in a positive light. With this Zechut, we will hopefully dance in the streets of Jerusalem on this Tisha B’av.