Pleasure-seeking and distraction are the hallmarks of the culture in which Montag lives. Although these may sound like a very self-serving set of values, the culture is not one that celebrates or even tolerates a broad range of self-expression. Hedonism and mindless entertainment are the norm, and so long as the people in the society of Fahrenheit 451 stick to movies and sports and racing their cars, pursuits that require little individual thought, they're left alone by society.
However, whenever individuals start to question the purpose of such a life, and begin to look for answers in books or the natural world and express misgivings, they become threats. Their questions and actions might cause others to face the difficult questions that their culture is designed to distract them from. For that reason, in the society of Fahrenheit 451 people who express their individuality find themselves social outcasts at best, and at worst in real danger.
Clarisse McClellan represents free thought and individuality. She's unlike anyone else Montag knows. She has little interest in the thrill-seeking of her peers. She'd rather talk, observe the natural world firsthand, and ask questions. She soon disappears (and is probably killed). Fahrenheit 451's society is set up to snuff out individuality—characters who go against the general social conformity (Clarisse, Faber, Granger, and Montag) do so at great risk.