In the novel "Great Expectations" by Charles Dickens, why is Joe's meeting with Pip in London a significant turning point in the novel?

When Joe visits Pip, Pip immediately discovers that Joe is intimidated by his new life. Joe is uncomfortable in Pip’s world and informs Pip that he will not visit him, that if Pip wants to see him he must come to his home where Joe can be himself and they can revert to their previous relationship. Pip fails to do this, however, when he returns to Rochester to visit Miss Havisham. Pip cares for Joe, but he is uncomfortable around him and dislikes the reminder of where he has come from. It is a sad note in the relationship and one that Pip is well aware of, one that leaves him filled with guilt.