Why did Jonas leave the community in The Giver?

Jonas leaves his community when he finds out that Gabriel is going to be released.

When Jonas finds out what release really means, he is shocked.  He has no idea, like everyone else in his community.  He thinks his community is pretty much perfect.  All he knows about release is that it is final.  Release of a newchild is uncommon and sad, because newchildren are babies.

Release of newchildren was always sad, because they hadn't had a chance to enjoy life within the community yet. And they hadn't done anything wrong. (Ch. 1) 

Jonas knows that release of a newchild is one of the only occasions when release is not a punishment.  (The other occasion is release of the elderly.)  In a way, however, it is a punishment.  Newchildren are released for not growing fast enough or otherwise meeting the community’s expectations, requirements, and targets.  This is what happened to Gabriel. 

Gabriel was a newchild that Jonas’s father felt “wasn’t doing well.”  He requested to bring the baby home at night, because in Jonas’s community all babies were raised in an institutional setting for the first year and Jonas’s father felt that a home environment with extra nurturing might be better for him.

Jonas is selected Receiver of Memory, and with this assignment comes special training.  Part of this training is information about the way life used to be before Sameness.  Jonas realizes that people used to care about each other and used to have feelings.  He experiences family and love.  He also experiences hardships such as war, hunger, and death.

When Jonas sees his first release, on video, he releases that release means death.  He sees his father conduct the release, which is the release of a newborn twin by lethal injection.  Having seen death through the memories, Jonas is horrified.

"So," Father went on, "we obviously had to make the decision. Even I voted for Gabriel's release when we had the meeting this afternoon." 

Jonas put down his fork and stared at his father. "Release?" he asked.

Father nodded. "We certainly gave it our best try, didn't we?" (Ch. 21) 

Jonas’s father has no clue what release really means, no matter how many times he has done it.  Only Jonas really understands.  When he finds this out, he decides to escape and take Gabe with him. He and The Giver had been planning an escape, but Gabe’s release made it necessary to move up the plans.