Though Ray Bradubury's Fahrenheit 451 is a profoundly disturbing book in many ways, the repetition of suicide is perhaps one of the book's most chilling features. Even more disturbing is the fact that so many characters commit suicide in Bradbury's dystopia because they cannot think critically or independently.
Consider two of the characters who commit suicide in the novel. Mildred, Montag's wife, is a typical, soulless member of future society, as she spends most of her time watching TV and generally wasting her life on meaningless entertainment. When she does try to overdose on pills, it's suggested that she simply forgot how many she had taken, thus leading to an inadvertent suicide attempt. However, thinking about this situation more critically, it's possible to assume that Mildred, despite her apparent complacence, tried to kill herself because she was trying to escape a life devoid of intellectual stimulation. Likewise, consider the anonymous woman who elects to burn along with her books, rather than losing them. Instead of losing her beloved literature and facing a life of intellectual slavery, this anonymous woman chooses death.
In both these situations, Bradbury's characters face a world full of meaningless entertainment designed to placate the masses and discourage independent thought. Ultimately, the prospect of living in such a world drives Bradbury's characters, whether they are chronically bored or in danger of losing their intellectual dignity, to suicide.
People commit suicide so often in this society because they are only made to believe they are happy when they really have no substance or purpose to their life. As soon as their minds are allowed to wander they come to the realization that their life holds virtually no meaning and it becomes so overwhelming that they do things like Montag's wife and take an overdose of pills. We know it happens often because the technicians who come to rescue his wife are not trained medical professionals, they are very simply techs who have been trained to operate the machine. This society has extracted meaning from the world by limiting the scope of expression, especially that of the written word.