מדרש על בבא בתרא 31:4

ז He said to him, "Why is it is that on a person's body every follicle has two hairs, but on their head every follicle has one hair?" He said to him, "The Holy Blessed One created on the head every follicle with one hair, but if They had created two in every follicle, both their eyes would be darkened." (Bava Batra 16a) And so too, for the world- if there were two raindrops in each 'follicle' the world would be destroyed and it would be more than the Flood. And in a place where the Holy Blessed One has created an affliction, They have created parallel to it its cure." He said to him, "How fortunate you are, my son, that the Holy Blessed One has revealed so much to you."