Why is it important to Guy's character to have Clarisse ask if he is happy?
The fundamental question of whether or not Montag is happy with who he is and what he does strikes at the very heart of his being. The question "Are you happy" demands a reflection about one's place in the world. While Guy sees Clarisse as "odd," he is not ready to engage in the level of reflection that her question triggers. Even before he can answer, she runs away, leaving him to answer the question to himself. This moves him into a realm of reflection that he never had to address before and one that he finds himself ill- equipped to understand. While Clarisse thinks "a lot," it is evident that Montag does not. It becomes disarming for him to do so. In having to reflect about Clarisse's question, it becomes clear that Montag is trying to justify himself to himself and no longer to her:
Of course I'm happy. What does she think? I'm not? he asked the quiet rooms. He stood looking up at the ventilator grille in the hall and suddenly remembered that something lay hidden behind the grille, something that seemed to peer down at him now. He moved his eyes quickly away.
In asking "quiet rooms" the issue of his happiness, Guy has already begun the process of changing. Bradbury shows internal reflection as a part of consciousness that cannot be reversed once it starts. The simplicity of the question causes Montag to have to explore aspects of his being that he never did before. In doing so, a change that compels Montag to question reality and his place in it more than ever before transpires. This reflection is triggered from Clarisse's question. It starts Montag down a spiral of reflection where embracing something new and rejecting the Status Quo are the only plausible options for him.