The problem with Pip is that, while very secure in the belief that he is of the utmost importance to the world, he is incredibly susceptible to the thoughts and opinions of others. No matter where he goes, Pip judges himself based on the actions of those around him, as well as their opinions of him; when living with Joe and Mrs. Joe as a commoner, he judges himself against their lack of manners and Miss Havisham's perceived grandeur. When living in London, he is bombarded by the scorn of high society and the unavoidable fact that he is not, in fact, of their class. This causes him to readjust his character in order to play to this newer and better stage. However, the very fact that Pip tries so hard to "fit in" makes him even more of an outsider, because his actions are so obviously an attempt to tear himself away from his past that they are crass and almost pathetic. 

His treatment of Joe is despicable, even though he once associated Joe with goodwill and kindness; Pip doesn't want this tangible evidence of his common beginnings messing up his new life, but even though he acts as though he has risen far above Joe on the social totem pole, he fails to gain the respect of those around him. Even as he is repeatedly humiliated in London, Pip completely fails to see the parallels between his treatment of those he sees as below him and the treatment he himself is subjected to. He doesn't seem to realize that Jaggers, Estella and co. look down on him just as much as he looks down on Joe and his somewhat grubby roots. This willful blindness is, of course, fed by his fervent belief that he and Estella are meant for each other, which makes it much harder for Pip to understand his position as the permanent outsider in her world.