Montag's interactions with Clarisse causse him to question his own happiness.
Montag begins to realize that society is not perfect. This causes him to acquire a thirst for knowledge and understanding.
Although he is a fireman, Montag starts to question his job (burning books). He finds himself drawn to their owners and wants to understand why they are willing to protect their belongings (books) at the expense of their own lives.
Montag realizes that people need history, that society had censored books in order to erase memory. This begins his quest to save books.
In his long discussion with Montag, Captain Beatty mentions the standard practice of immediately cremating the dead so society is not burdened with decaying bodies or memorials and the grief associated with them. Later, as Montag comes to realize the truth about his society, he recognizes fire as a form of oppression - a means of subduing the knowledge in books.