In Fahrenheit 451, why does Clarisse have such an effect on Montag?
Although he has lived happily in society all his life, some part of Montag is unsatisfied. Meeting Clarisse puts his dissatisfaction into perspective; he has already started to steal books, more out of curiosity than rebellion, but her interesting ideas on life and culture give him mental stimulation, which he cannot get from his wife or from television:
"Have you ever smelled old leaves? Don't they smell like cinnamon? Here. Smell."
"Why, yes, it is like cinnamon in a way."
She looked at him with her clear dark eyes. "You always seem shocked."
(Bradbury, Fahrenheit 451, Google Books)
Montag's slow growth into an individual is spurred by Clarisse, and so he feels a connection with her that he doesn't even feel with his wife. When she vanishes -- probably dead in a hit-and-run, but Montag never finds out for sure -- he feels alone, and only slowly regains his focus on trying to feel and learn real things, not just what he sees on TV.
Montag likes her new way of thinking. She is the first to begin to question things. He suddenly realizes that he is not happy, and becomes more curious about books. He likes the past better than the present, because they had more freedom. He wonders why the government is hiding the information in the books from people. They do that because the government wants everyone to be made equal.