In Fahrenheit 451, what is the major factor that brings the society to what it is now?

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.