Why was the Song of Songs written?

God didn't chose to tell us. All of the Bible was written to guide mankind. "As His divine power has given to us all things that pertain to life and godliness, through the knowledge of Him who called us by glory and virtue" (II Peter 1:3). The Song of Solomon, or the Song of Songs, deals with one aspect of life: love. Thus important lessons can be learned from its pages. "All Scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness, that the man of God may be complete, thoroughly equipped for every good work" (II Timothy 3:16-17).s

See also:

Questions and Answers regarding the Bible
The Greatest Love Song Written