I guess that is my question... What are Sir William's motives? Is he polite and considerate of others (giving them the info about Bingley he knew they would want, for example) because he wants to give them pleasure, or is he acting like someone from "court" who plays the part but whose motives are selfish so he is the one who is pleased?

I agree with Julie W above that he really took this "Sir" thing too far. He does seem to be supercillious even though the narrator says he isn't. (I guess I am trying to understand if the narrator is being sarcastic or true. Maybe I am reading too much into this.)