From page 107 of his classic On Being a Theologian of the Cross, in reference to Thesis 26 of Martin Luther’s Heidelberg Disputation:

“We see that the law simply cannot bring into being what it commands…The law says, ‘Thou shalt love!’ It is right; it is ‘holy, true, good’. Yet it can’t bring about what it demands. It might impel toward the works of the law, the motions of love, but in the end they will become irksome and will all too often lead to hate. If we go up to someone on the street, grab them by the lapels and say, ‘Look here, you’re supposed to love me!’ the person may drudgingly admit that we are right, but it won’t work. The results will likely be just the opposite from what our ‘law’ demands. Law is indeed right, but it simply cannot realize what it points to. So it works wrath. It can curse, but it can’t bless. In commanding love law can only point helplessly to that which it cannot produce.”

A great cover: