Why has Montag suddenly reflected on Clarisse's outlook on life?

According to the text, Montag and Clarisse have multiple conversations while he is traveling to and from work. Clarisse questions Montag about his life and marriage, and even seems to perceive him as unhappy. In turn, Montag begins to question his own life, happiness, and relationship with the woman he doesn't seem to know anymore. This questioning increases after he is out on a call at at house where an older woman's home is burned down with her in it.