At the outset of the novel, Guy Montag has been a fireman for ten years. This is different from a fire fighter because his job is to light fires (to burn books), rather than to fight them. For the duration of his ten year career, Montag has (for the most part) been happy, believing that he contributes to the overall happiness of society. He has pursued the goals that society sets ahead of him, though his theft of books over the years indicates that he's had doubts. The events of the novel, beginning with his meeting with Clarisse McLellan, are the catalysts for rebellion.
Guy Montag has been a fireman for ten years.
Fahrenheit 451 is a novel about independence of thought, and the exposition of its narrative clearly presents the lack of worthy thoughts that Montag's wife and so many others display in a futuristic society. Though Montag has been stealing books for years, it is his unexpected encounter with Clarisse McClellan, whose face contains in it a "gentle hunger that touched over everything with tireless curiosity," that causes him to begin seriously questioning his society. Then he comes to realize how he has lost a part of his humanity.
After the curious Clarisse asks Montag if he ever reads any of the books that he burns and if he is happy, Montag is shaken. "What a strange meeting on a strange night," he remarks as he walks home. He marvels at how strong Clarisse's sense of self-identity is. Clarisse's power contrasts sharply with the personalities of Montag's wife, her friends, and many others.
It is at this point that Montag's life begins to change; he has had his natural instincts, so long repressed, now piqued.
In Fahrenheit 451, Montag has been a fireman for ten years. This is revealed in Part One of the book when Montag first meets his neighbor, Clarisse McClellan. She asks him about his job, specifically how long he has worked as a fireman. He replies that he started working in this profession ten years earlier, when he was twenty.
Through this conversation with Clarisse, the reader learns the extent of Montag's dedication to his job. He claims to have never read a book and recites a short rhyme to Clarisse about burning particular authors on particular days of the week. This meeting with Clarisse, however, will have a profound effect on Montag's view of his profession, making him completely re-evaluate the fireman system and the value of censorship more generally.