The Song of Solomon is not a book that many people reference. In fact, many people do not know exactly what to do with the book as it depicts the amorous love between two people and at times is it pretty graphic. However, if you read the book closely within the canon of the Bible, we can say a few things regarding the theme.
First, the idea of love is not something that is foreign in the Bible. For example the book of Hosea is a book that deals with the topic in a oblique way. Hosea is bid to marry a prostitute, Gomer. Hosea pursues after her with his whole heart, even when she flees. This expresses gracious love. Most importantly one of the broader themes of the Bible is God's love in the context of marriage.
Second, the Song of Solomon should be looked at a deeper level. In other words, it is an allegory which underlines the love that God has for his people. It really is about the passionate pursuit of God for his people and the joys that people can experience when they find the love of God.
Perhaps the most famous quote in the whole book comes from Song of Solomon 6:3. "I am my beloved’s and my beloved is mine." It really is a book of joys of love.