why is jack hospitalized after a conversation with buckly in the lovely bones

Jack ends up hospiatalized because he needs a knee replacement. He's been rather out of control, he's trying to prove that Mr. Harvey is Susie's killer (no one believes him, not even his wife), and during a lte night walk in the cornfields because he sees a light, and he happens upon Clarrisa whi is waiting for her boyfriend. The boyfriend, attacks Jack and beats him. breaking his kneecap.