Why did Jonas leave the town in The Giver?

Jonas’s decision to leave begins when he and The Giver
start to explore what would happen to the community if he left.  In chapter 18, Jonas
asks what would happen if he was in an accident.  They have discussed the pain the
community went through when Rosemary was released and the memories were returned to the
community.  Jonas notes that he has many more memories than Rosemary had at the time of
her release, so the chaos that resulted would be greatly
magnified. 

The Giver becomes thoughtful, and replies, “If
you floated down the river, I suppose I could help the whole community the way I’ve
helped you.  It’s an interesting concept.  I need to think about it some more” (145). 
This is the first time that Jonas leaving becomes a
possibility.

When Jonas views the video of his father
killing the newchild in chapter 19, he has a violent reaction.  For the first time, he
realizes how terribly wrong things are in his community. He begins to believe that the
only way they can recover and regain their humanity is by recovering their memories. 
This means that not only can Jonas leave, but he has to in order to save them from
themselves.  The Giver tells Jonas, “They can’t help it.  They know nothing. 
… It’s the life they live.  The one that was created for them” (152).  Jonas
considers whether or not it is their responsibility to fix this.  When Jonas asks The
Giver why they should care about the community’s lack of humanity, he realizes that, “of
course they had to care.  It was the meaning of everything”
(156).

Jonas is excited about the plan, and he and The
Giver continue to work on it.  But in chapter 21, Jonas learns that Gabriel is scheduled
to be released.  He cannot allow this to happen, especially since he has been giving
memories to Gabe.  The plan has to be moved up, and Jonas escapes before he is
completely ready.