What experience changes guy significantly in the first chapter of Fahrenheit 451?

Throughout the novel Fahrenheit 451, Guy Montag goes from a relatively "typical" fireman to a man on the run. In the story, Guy meets his seventeen year old neighbor Clarisse McClellan, she questions Montag about his life now and the life he has always lived. Near the start of the book Clarisse asks Montag, "Are you happy?" Clarisse's question influenced Montag in many ways, causing him to wonder about his life. After speaking with Clarisse, Montag starts thinking about his wife, Mildred, and whether they really love each other or not ...