Why must Jonas have courage and bravery to be a Receiver?

I can think of a few reasons why Jonas needs to be brave in order to be the next Receiver. The first reason that Jonas is brave is that he is chosen to do a job that he knows nothing about. Before being given a final job, each child spends time volunteering in various positions. The child has a chance to learn about and participate in various working fields. Jonas has a fairly good idea about what many jobs in his community involve; however, he doesn't know anything about being the Receiver. The unknown can be scary, and Jonas needs to be brave to step into that role.  
Jonas also needs to be brave because he's alone. He's the only child chosen to be the next Receiver; this is the nature of the position, but it's still scary to take on an unknown task by yourself. To compound the issue, the Giver's rules forbid Jonas from discussing what he learns with anybody. Jonas is literally taking on the emotional load of an entire community by himself, and he can't even talk about it with a friend. He needs to be brave.  
Finally, Jonas needs to continue to be brave because he learns what happens to Rosemary. She was the previous Receiver, and she couldn't cope with all of the memories and feelings that she was being given. Even the Giver couldn't give her enough happy memories to undo the emotional toll that the pain and suffering memories had put on her. 

The Giver continues, "I backed off, gave her more little delights. But, everything changed, once she knew about pain. I could see it in her eyes."

The emotional weight eventually became too great for Rosemary, and she asked to be "released" from the community. Her death affected the entire community because her knowledge was released to the community as well. The Giver tells Jonas that "It was chaos." 

"They really suffered for a while. Finally it subsided as the memories were assimilated. But it certainly made them aware of how they need a Receiver to contain all that pain. And knowledge." 

Jonas needs to be brave because he knows that the previous person to hold his position failed and asked to be killed, and he knows that his death would cause problems for an entire community. Jonas needs to be brave because he has a huge responsibility in the community.