Perhaps because God, through his writers and prophets, saw today’s sexual depravity and wanted to depicted exactly WHAT romantic love entails, and what it DOESN’T.
The Douay-Rheims Bible calls this book “The Canticle of Canticles”, a title which IMO was designed to ward off all but the most determined readers of Scripture.
I didn’t read it until high school & it seemed then like something out of the Rubaiyat or the Thousand & One Nights.