I was looking at Nygren's Agape and Eros again the other day and it occurred to me that for all its flaw it got something right, namely that divine love is a great mystery. It is a mystery because it is not motivated by the object as in eros or in what Nygren refers to as the "caritas synthesis" of Augustine. For Catholic thought, as for Platonism before it, God loves things because they are desirable. He first loves himself because he is supremely desirable and then he loves other things insofar as they are like him. By contrast, for the God of the Bible, the object of love is utterly undesirable, and yet he loves it. This is mysterious. Why love a thing which has nothing inherently attractive to it?
Also, I think this shows that the Plato's eros and the caritas synthesis are more or less rationalizations of God. They are in effect attempts at rationalizing the mystery of why God loves. As we know from Luther, law and reason always go together. Law then becomes the means whereby the divine mystery of love is made explainable- i.e., we earned it! But that's not how God works. His love and promise in the gospel are every an incomprehensible mystery, which he gives freely for no apparent reason.