After meeting Clarisse, Guy Montag begins to question things in ways he's never done before. Previous to this encounter, Guy simply went about his business as a fireman and did as he was told. In their first meeting, Clarisse tells Montag that she had once heard that firemen used to put fires out, rather than start them. Montag laughs. Clarisse replies:
"You laugh when I haven't been funny and you answer right off. You never stop to think what I've asked you."
This is a key and blatant moment in the early part of the novel where Clarisse clearly challenges Montag to "think" about things. As she leaves Montag, she asks him if he is happy. Ordinarily, Montag would not give such a thing, or anything, a second thought. But Clarisse sparked some curiosity in him and he begins to question if he is happy. As far as Montag realizes at this point, no one has ever challenged him to think in this way:
How rarely did other people's faces take of you and throw back to you your own expression, your own innermost trembling thought?
As the novel continues, Montag's curiosity grows. He continues to question his life, his role as a fireman, and the supposed illegality and evil of books. He stockpiles books he steals from fires and reads. He questions his wife's lifestyle and realizes any significant connection they may have had is lost. Eventually, Montag's curiosity gets him into trouble with Beatty and higher authorities. But by this time, he has passed the point of no return as he becomes determined to question things, gain more knowledge, and find greater significance in his life.
HE realized that books were not all evil and he joined the band of book "hobos" in the hope to spread knowledge and get people aware that books aren't bad.