Clarisse is a second-generation individualist, and so very strange to Montag, who is a third-generation fireman. Clarisse sees the strangeness in society for what it is, the suppression of free thought by the government. Montag, in contrast, is uneasy about society but doesn't know why. Because Clarisse sees past all the things that Montag takes for granted, she opens his eyes to some of the stranger and more anti-social habits of the modern society:
"You laugh when I haven't been funny and you answer right off. You never stop to think what I've asked you."He stopped walking, "You are an odd one," he said, looking at her. "Haven't you any respect?"
(Bradbury, Fahrenheit 451, Google Books)
Clarisse serves as Montag's human portal into individual thinking, where he had trouble thinking about his personal experiences in any way other than the societal norm. Now, with the opening of individual thought given to him by Clarisse, Montag is better able to understand exactly why so many things in society make him uncomfortable, why he has been stealing books, and why the conformity he views in his wife Mildred is so wrong.