Clarrise asking Montag if he's happy sets off a chain reaction that changes his life completely. This question makes Montag start to question his life and the society around him. 
Remember Montag we're the Happiness Boys...We stand against those who want to make everyone unhappy.
Mildred's overdose gives a perspective on how people feel in thissociety. Even though the society Montag is in filled with entertainment, people don't feel happy enough to keep living
Go ahead now you, second-hand litterateur, pull the trigger.
The woman who kills herself with her books makes an impact on Montag's perception of the books. He realizes now that the books must be more important than they seem if a person will die for them.
BBeatty visiting Montag gives us an inside look into his mind. We learn that he is educated but resents books because of the unhappniess they bring. 
Montag killing Beatty is the first step into him fully changing into a different man. This is the event where Montag finally does something on his own accord.
Montag jumping into the river is the final step of him changing as a person. Montag jumping into the river is symbolizing that he is washing away all his past mistakes and turning into a man that praises books instead of perishing them.  
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