How does Ray Bradbury keep Fahrenheit 451 from being pessimistic?

Ray Bradbury prevents his narrative from becoming too pessimistic because there is a subtle mood of hope generated through characters and incidents in the plot.
In other dystopian works, such as George Orwell's 1984 and Aldous Huxley's Brave New World, the protagonists feel a terrible sense of aloneness and alienation. But, throughout the narrative of Fahrenheit 451, Montag can communicate with such people as Clarisse, professor Faber, and later Granger and the others who memorize books. Because there are those with whom he can share ideas and communicate, Montag does not suffer as badly from alienation as do other characters in dystopian worlds. He also escapes from his society, whereas the main characters of the other works do not. One critic writes, "Montag’s conversion to reading is significant in that he suddenly finds himself in light rather than darkness."
Montag enters no terrible Room 101 as does Orwell's Winston, nor is he sent to desolate geographic areas as is Huxley's Bernard. Montag happily joins a community with people who are like him so he will find meaning in his life. These people are the light of the future, the "sea of faith" of which Bernard merely hopes to be a part.