One the dominant themes of Great Expectations is what might be called "the true measure of a man." When Pip becomes wealthy from his unknown benefactor, he goes to London and becomes puffed up with an exaggerated and false view of himself and others. He disdains his former life and those of his acquaintances who no longer meet his social standards.
When Joe comes to visit Pip, Pip is ashamed of him and his workingman manners and dress, although he has enough conscience left to at least try to hide it, at first. But Joe, though uneducated, is a shrewd judge of people, and he easily sees that he isn't welcome and that the Pip he once knew is mightily changed.
However, this doesn't change goodhearted Joe's love of Pip, and when Pip needs him, Joe is there for him. Just as in the same story when the outside of Magwitch, the convict, is shown to not be the real man beneath, Dickens continues exploring his theme of what truly is valuable in human character.