What's with this book of love poetry hidden in the middle of the Bible? Maybe it's a reminder to take advantage of the bodies God made for us, to find joy in pleasure and beauty, and understanding. To look back at the beauty of young love and forward to love beyond this life and say, "oh, what it is to be young and in love," and use it to speak truth about what it is.
One note: I slightly misstated the Karl Barth reference--he was slightly more nuanced, and said that nearly all love and marriage is to answer the question of posterity, except in the Song of Solomon.

Image: Magritte's "The Lovers 2"