At the beginning of Bradbury's Fahrenheit 451, Montag (the protagonist) has no concerns at all. He shows up for work each day, mindlessly burning down homes that contain books. He does not question his actions or the laws of society.
The first element of the story that makes Montag look more closely at his behavior and society's norms is Clarisse McClellan. She asks him questions about the world, about things he remembers, about what they miss in their daily lives, and—perhaps most importantly—she asks Montag what he thinks...about anything and everything. As much as he tries to resist the implication she makes, he realizes that he is not in control of his own thoughts. From this point on, he will begin to question the status quo.
However, when Montag witnesses a woman's willingness to take her own life rather than live without her books, he is shaken to the core. She will not allow him to save her, and her death destroys his casual view of the world and his place in it. First...
The woman knelt among the books, touching the drenched leather and cardboard, reading the gilt titles with her fingers while her eyes accused Montag.
"You can't ever have my books."
This is ironic in that the firemen don't want her books, or that she have them either. However, she will not even give over their possession to allow them to burn them: she chooses to do this herself, and Montag is devastated.
"You can stop counting," she said. She opened the fingers of one hand slightly and in the palm of the hand was a single slender object.
An ordinary kitchen match.
As the woman and the books and house burn, Montag is changed forever. From here on, Montag's concerns are society's laws: laws that control what people are allowed to own (books), think (they shouldn't), and act. Society has wall televisions with controlled viewing that robs people of the desire to use their imaginations or question anything.
Montag is concerned that the world has become desensitized: Mildred is so out of it that she overdoses on pills and is totally unaware of it. She slips at night with devices in her ears that deaden her to her surroundings. She becomes an automaton—a robot.
Montag is enthralled by Clarisse McClellan's perceptions and ideas, which open an entirely new world to him. He is once again devastated by a world that is moving so fast and with so little regard for the individual, that a speeding car kills Clarisse and no one seems to notice. There is no police investigation: no one is held accountable.
And Montag resents burning books and the houses that hold them. He starts to hide books in and around his home and secretly read them. He makes contact with Faber, another man who loves books. Finally Montag escapes in order to join others who want to change the world, fashioning a new society around books: the information they contain, and the freedom of thought they represent.
Fire takes on a new form: it is used to destroy at first. At the end, it is used to warm the people leaving the city. It represents light and the knowledge that comes with it...and holding on to that knowledge. Granger notes:
We pick up a few more people that remember every generation.
Montag is concerned with being a part of this kind of fire—being one who remembers.
Montag has few concerns at the start of the novel; by the end, his concerns are reflected in his desire to help change the world.