Why did Silver decide to ally himself with Jim?

Long John Silver says that since he was elected Captain he will protect the boy because "I never seen a better boy." In response, the rest of the pirates leave the house and have a council outside. While they are gone, Silver tells Jim that he decided to save the boy only because he was courageous enough to stand up for the boy. Silver vows to Jim that he will save the boy if he will be a witness and save Silver from hanging when they return to civilization. Jim agrees and the shake on the agreement.