One would think that ninety-eight curses should be sufficient warning to impact the nation. One has only to read the curses specifically, and his hair would stand on end from fright. Yet, it appears that Hashem seems to “throw in” one more curse for “good measure.” In case there might be something that was missed, any sickness or plague that was not specifically mentioned, it too will be included.
Horav Shimshon Pincus, zl, explains that the Tochachah is more than an admonition. It is referred to as Divrei HaBris, Words of the Covenant. Indeed, the tragedies, the misfortunes that are destined to occur as a result of our egregious behavior, are in themselves part of our covenant with Hashem. Without the threat of punishment, our bond with Hashem would be severed. Without the fear of serious retribution, without realizing this fear, we might disconnect from the Torah way of life and eventually leave Hashem altogether. It is only as a result of this painful rod that we keep on returning. We know what happens when we leave; therefore, we keep on returning.
What happens after we have, Heaven forbid, exhausted all ninety-seven curses? What will become of us? How will we be able to maintain the Divrei HaBris? This is why the Torah adds one last curse, throwing every imaginable punishment at us. We do not have to worry. Even after Hashem has delivered all of the previous punishments, there is still more. He will never let us go. Regardless of how much we have suffered, Hashem will keep the sword over our collective heads to ensure our eventual return. These are dreadful curses, but they are the admonishment of a loving Father, who will do whatever it takes to see to it that His children never leave.