Because of the enormous population, and the cultural acceptance of sitting in one's own house watching television instead of socializing, Montag realizes suddenly that he is all alone in the world. Even his wife, Mildred, barely interacts with him, preferring to remain in her parlor and watch the television screens that cover three entire walls. His friendship with Clarisse is his first that is based on personal interest instead of cultural norms; he only interacts with his fellow firemen because they work together. After two handymen -- not even doctors! -- come to save his wife from an overdose, Montag reflects:
There are too many of us, he thought. There are billions of us and that's too many. Nobody knows anyone. Strangers come and violate you. Strangers come and cut your heart out. Strangers come and take your blood. Good God, who were those men? I never saw them before in my life!
(Bradbury, Fahrenheit 451, Google Books)
Montag's epiphany is that all the people in the city, even though they live so close together, are entirely alone. The government discourages personal relationships and communication because that leads to conversation and individual thought; Clarisse comments that everyone says the same things, expresses the same opinions, and no one has any non-typical thoughts. Montag doesn't know the people on television, doesn't really know his neighbors, and discovers that he barely knows his wife. As the novel progresses, Montag finds himself more and more alone, with no one to talk to about his growing uncertainty.