It is believed that it was written on the occasion of King Solomon taking a new bride–his wooing of her and her place within his affections. That’s the plain, literal answer. As LilyM cited, it has spiritual depths, as well. Besides being about God as the beloved and the individual soul as the object of his love, it is about Christ and his Church. The love he has for his Church, which is his Bride. For Jewish believers it is about God’s love for his Virgin Bride, Israel. Bible scholars believe we have but a fragment of a longer love poem, a portion of which has been lost in antiquity. Still, what we have has profound in meaning for both Christians and Jews.