Having seen Emily's raw emotion after meeting with Martha Endell, David feels that he has witnessed something extremely private. He doesn't even want to talk about her with Steerforth.
David gets a letter from Miss Betsey asking what he wants to do next. After parting from all the Yarmouth Peggottys and Mr. Omer and his family, Steerforth and David leave Yarmouth.
David tells Steerforth about his aunt's question: what should he do with his life? Miss Betsey has suggested that David become a proctor (in a word, a lawyer)