Bradbury's main message is that a society that wants to survive, thrive, and bring its people fulfillment must encourage them to wrestle with ideas. He indicts a society that puts all its emphasis on providing people with a superficial sense of happiness.
Beatty sums up the dystopic vision Bradbury opposes when he defends the importance of the book burning firemen:
The important thing for you to remember, Montag, is we're the Happiness Boys, the Dixie Duo, you and I and the others. We stand against the small tide of those who want to make everyone unhappy with conflicting theory and thought. We have our fingers in the dyke. Hold steady. Don't let the torrent of melancholy and drear philosophy drown our world.
Bradbury, in contrast, argues that without some conflict, thought, melancholy, or philosophy a society is doomed. He illustrates this in two ways in the novel. First, average citizens like Mildred live lives of quiet desperation. Mildred gets so bored with a constant of diet of television drivel and a lack of meaningful activity that she attempts suicide. The "happy" society Beatty defends is filled with bored adults and violent teens who spend their time watching vacuous television shows or speeding around in cars. People exist in a dehumanized way rather than feeling fully, vibrantly alive.
Second, at the end of the novel, this society is literally destroyed in a nuclear war. By showing the consequences of banning them, Bradbury highlights the importance of books and critical thinking skills to a healthy society.