What causes Montag to realize that he is not happy?

When Montag meets Clarisse, she's like a breath of fresh air.... unlike anyone he's ever known. It's the way that Clarisse views the world that causes Montag to question his own happiness, it's also a question she puts to him directly. Although their encounter is brief, Clarisse's love of nature, conversation, and family make him question his whole life. He comes to yearn for what she describes and comes to understand that he is unhappy.