17th of Tamuz again… why do we fast for something that happened in the past?  
Okay, we could say we are fasting so as to prevent future problems and avert negative situations. That is true as well.
But the fact is, for a Jew, it is not really the past. Because the destruction of the Beit Hamikdash is an ongoing situation.
As it says in Torah, if our generation does not witness the rebuilding of the Beit hamikdash it is as if we destroyed it.
And not only are we talking about the physical Beit Hamikdash. There is a spiritual mikdash within each and every Jew: that holy place that is pure, untainted by the outside world, untainted by impurity or unholiness. But that part inside ourselves can become eroded as well, part of it can be destroyed, albeit temporarily.
and so we need to rebuild our own inner sanctuary and that hastens the rebuilding of the holy Temple in Jerusalem. 
so we fast because we want these three weeks to be changed into times of simcha. Of open revealed good.
It is interesting to note that the number 17 equals bgematria to tov (good). So being that the fast begins on the 17th means it is really a hidden good. And may that good become revealed good with the coming of Moshiach speedily in our days.