there are many layers to the day, and reasons to observe it. according to tradition, one of the reasons the Temple was destroyed was due to the hatred the Jews of that period had for each other. So, we can fast on the day and reflect upon how that reason is still so prevalent today, and what we can do to lessen it.
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from the article: Anshel Pfeffer / It is wrong to fast on Tisha B'Av