What are the "itch" and "scratch" of which Beatty speaks in Fahrenheit 451?
According to Bradbury, the people must not be "upset and stirred" because it might cause them to think and learn and act against the government, which controlled all information "for the good of the people."
Although Fahrenheit 451 was written at the height of the Cold War, Bradbury's message is frighteningly applicable to contemporary culture. Beatty talks about a society that strives to offend no one and becomes one wherein intellectualism becomes a "swear word" while everyone runs amok being "happy." Contemporary culture also seeks after happiness and security before knowledge and liberty, which makes this book as relevant today as is was in 1950.
The "itch" Beatty speaks of is curiosity. Beatty referenced this tendency also when he complained that Clarisse wanted to know "why" things worked instead of "how." Curiosity indicates a desire to learn and to know, which, in Bradbury's society indicated dissatisfaction with the status quo. To "scratch" the itch was to read. Beatty discouraged that by telling Montag that books were full of nothing.
The hound was designed to sniff out the nonconformists--the thinkers, the readers, the philosophers. Those who think change society. In a highly controlled society like the one Bradbury created, thinkers were a threat. Montag's mind was opening to the possibility that there was more to life and the world than the cultural controllers wanted people to know. The hound instinctively knew that Montag was changing from a mindless fireman to a curious philosopher. That made him a threat to society, and so the hound "threateningly extended its silver needle toward Montag" as a warning.