The Fire Chief explains to Montag that the books have to be burned because the ideas in them are contradictory and hurt people's feelings. They are insulting or make certain groups of people feel bad--the Jews, the blacks, the homosexuals, etc. Society has decided that firemen are needed to get rid of these offending publications so that people all over the world can be happy. They then immerse themselves in pure entertainment which is not anything like Carlos Mencia...they go out of their way to make people laugh instead of offending anyone.
In doing so, those who are teachers, professors, writers, publishers and readers are out of jobs and hobbies. This creates dissention and unhappiness in these groups, and also fear, since many of them hide their precious books which are now illegal.
To be succinct, people become hooked on pure entertainment and stop asking questions.
The culture becomes so saturated with living vicariously through entertainment that they fail to take part in the world around them. This, of course, is just want the government wants. The fewer people who take part, the fewer people there are to voice opinions about how things should be.
When the public stops thinking and questioning, they just blindly follow along. One reason they stop reading is because books, as Bradbury suggests, cause people think and question. They too often reflect a world that is unjust and unhappy.
This leads the government to dictate to people what makes them happy - like the fun parks, the shows on the TV parlors, and the government controlled curriculum in schools.
Montag notes how people, like Mildred, might think they are happy when they really are not (look at the high suicide rates or the kids who murder and run down people). They simply have no healthy release for their frustrations. Again, Bradbury would argue that books allow this.
When the city is destroyed, the men decide to return and build a factor to manufacture mirrors. Now they aren't being literal. They are simply noting the importance of people being able to look at themselves honestly and to accept their flaws. When they do this, they can begin to correct those flaws and have a change at making a better world.