Why did Silver want Jim to join him?

In Chapter 28, Silver wanted Jim to join him because is a way he needed him. Jim was young, he had access to the Captain's quarters, and he worked amongst the people Silver was working against. He hoped to make Jim his ally, and he likened Jim to himself as a young man, telling him they would have made a great team. He then asks him to join the pirates.

In response to Silver, Jim confidently explains that it is he who has been the downfall of the pirates - through the apple barrel, the cutting the schooner loose and killing the men on board, and bringing the ship to a place that the pirates will never find her. Jim concludes with, "I no more fear you than I fear a fly." He tells the pirates that they may choose whether to kill him or to spare his life, but if they let him live, he will try to help keep them from hanging. When Jim finally finishes his diatribe, none of the pirates move. Thinking that he is dead, Jim tells Silver to tell Dr. Livesy that Jim courageously stood up to the pirates and that Jim thought that Silver was the "best man here."

Some of the pirates begin to speak against Jim, and Morgan springs up and draws his knife in an attempt to kill them, but Long John Silver stops the pirates and says the 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.