In Stephen King's book "Rage," the quote reflects the duality of human nature. It suggests that behind every person who appears happy and composed, like Dr. Jekyll, there exists a darker, hidden side, akin to Mr. Hyde. This concept explores the complexity of individuals, illustrating how everyone possesses contrasting traits that they often keep concealed from others.

The phrase implies that anyone can harbor inner turmoil or darkness, which is a fundamental aspect of existence. By acknowledging this duality, the quote encourages readers to confront the less admirable parts of themselves, as they are an integral part of the human experience. The tension between these conflicting aspects is a central theme in the narrative, serving as a reminder of the struggles individuals face within themselves.