During a conversation between Montag and Faber, Faber says, "Those who don't build must burn. It's as old as history and juvenile delinquents" (Bradbury, 42). Faber is saying that those individuals who do not positively contribute to society are helping to destroy it. Faber's analogy corresponds to the function of the firefighters, who destroy books and prevent individuals from attaining knowledge. Montag, who is a jaded firefighter, feels extremely guilty about participating in the destruction of society. He visits Faber in hopes of altering the trajectory of his life and wishes to contribute to society instead of being a destructive force. Faber believes that authors, artists, and intellectuals "build" institutions and have an overall positive impact on society. According to Faber's views, individuals who do not give back to society either directly or indirectly contribute to its destruction, which explains his own feelings of guilt. While Faber does not destroy novels like the firefighters, he also does not go out of his way to prevent them from doing so, which makes him complicit in the destruction of literature.

Faber means that people who don't want to contribute to society often end up destroying it. 

Montag explains to Faber that Captain Beatty has read a lot of books—because of this, he's knowledgeable, and Montag wants to talk to him. If he does, though, he's putting himself and Faber at risk. Montag thinks back on his past as a fireman and explains to Faber that only a week ago he was pumping kerosene into the places he needed to burn and felt like it was fun. Now his world is radically different.

Faber says, "Those who don't build must burn. It's as old as history and juvenile delinquents." What he means is that human nature isn't static. People either create or they destroy—it's rare for someone just to sit and do nothing. Montag was a person who destroyed; his experiences changed him enough that now he's someone who creates.

Faber goes on to say that there's "some of it in all of us," showing that the human tendency to create or destroy is universal. 

This quote is from Part II of Farenheit 451  in which Montag seeks Faber's help in getting his bible reprinted so that he can turn one copy over to the authorities if he needs it. While he and Faber talk at the old professor's home, the curious Montag asks his new mentor to explain to him that which he reads. Then, Faber explains the what Montag needs to learn is truly around him; it is only that books facilitate knowledge by recording the events and experiences of life. And, when people read books, they devote time to thinking while they read and afterwards. However, in their society, people are not allowed this time because they are subjected to constant stimuli from technology that distracts and clouds their thinking. Faber contends that people need the time to learn and the right to use what they learn in order...