What causes Montag to change throughout Fahrenheit 451?
Montag's changes start before the beginning of the book itself; it turns out that he has been stealing and hiding books for months, maybe years. He's not sure why, or what about books appeal to him, but he knows that something in society is wrong. His first meeting with Clarisse triggers him to start examining himself and his relationship to society, and this allows him to start feeling uneasy about the powers wielded by the firemen. Later, when he meets with Faber, he is able to voice part of his concern:
"...We have everything we need to be happy, but we aren't happy. Something's missing. I looked around. The only thing I positively knew was gone was the books I'd burned in ten or twelve years. So I thought books might help."
[Faber said] "...It's not books you need, it's some of the things that once were in books."
(Bradbury, Fahrenheit 451, Google Books)
As Montag grows as an individual, he realizes that the concerns and focus of society have nothing to do with reality, or with the distress he begins to feel at the non-value placed on human life. The turning point comes when he is forced by Chief Beatty to burn his own books; after this, all his decisions are informed by his own individuality, instead of either society's pressures or his own need to hide his feelings. By the end of the book, Montag knows that he is of value to those who care to listen.