Rabbi Chaim ben Betzalel writes (Sefer Hachaim, Chelek 5, Perek 6) that the current, fourth exile is similarly to “kadachas” (an illness characterized by high fever), which the body has to fight off completely in order to be immune from a future attack. If the person takes medicine and stops the illness too early, it will only come back in a stronger form. So too, we must not bring the geulah before its time. “Even though he may tarry, wait for him” (Chavakuk 2:3).