Gedor Pirtzi Beven Partzi
On the seventeenth of Tamuz, the walls of Yerushalayim were breached. This is one of the reasons we fast on this day.
Throughout our exile, we, the Jewish people, have set up our own walls to stem the tide of being too influenced by the nations that surround us. The walls hold steady for a while, but the influences seep in little by little. Just when we think that a certain deviancy is pas-nisht for a Jew, we are proven wrong each and every time. This too should be reason enough to fast.
The only things that can patch up the holes is learning Torah and doing Mitzvot and Hesed. Those are the only remedies until Mashiah comes, at which time, he will be the leader that closes up the breaches.
Part of the liturgy for the seventeenth of Tamuz is: