17 Tammuz: Punishment or Love?

Externally, the fast day of 17 Tammuz is a matter of punishment. But the inner aspect of the fast -- which is the aspect of the fast that will be revealed when Moshiach comes -- is a day of joy and happiness and a festive time. Which means that it is not an instance of punishment which brings about the geulah, but rather that the fast itself is at its inner core a matter of joy and happiness. This reveals that even the aspect of punishment associated with the day comes from the Almighty's love for Jews, as expressed in the analogy of "a great and awesome king who himself in all his glory washes the filth from his only son due to his great love for him," as stated in the posuk "When the Lord shall have washed away the filth of the daughters of Zion" (Yeshayahu 4:4).

Likutei Sichos 18:313 (free translation)