In Chapter 27, of Great Expectations, why does Joe call Pip "sir"?

In Chapter XXVII of Great Expectations,
Pip has decorated and reupholstered furniture in his and Herbert's apartment
in London and even hired a servant boy so that the appearance of the place befits a
gentleman.  As Joe encounters Pip, he greets him
heartily,

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Which you have that growed...and that swelled and
that gentle-folked...as to be sure you are a honor to your king and
country.

He and Pip have a
warm conversation that is interrupted by the entrance of Herbert. When Herbert extends
his hand, Joe backs up from it, feeling not the equal of Herbert, whom he recognizes as
a young gentleman.  Lest Pip's standing with Herbert be mitigated by his acquaintance
with Joe who now becomes aware of Herbert's social class, Joe acts as though he is of
inferior social class to Pip, as well to Herbert. Besides, Joe senses his class
consciousness. 

When he departs, Joe explains to
Pip,

"Pip,
dear old chap....Diwisions among such must come, and must be met as they come....You and
me is not two figures to be together in London; nor yet anywheres else but what is
private, and beknown, and understood among
friends."

Joe tells Pip that
he is out of place in London, for he belongs on the
forge:

"You
won't find half so much fault in me if you find me in my forge
dress...."

This scene
exemplfies one of the themes of Charles Dickens who felt that society was a prison in
which people were cast into their social standing.  Since Joe has been brought into the
world of those of a higher class than he, Joe feels awkward; then, out of love for Pip,
whom he knows wants to be a gentleman, Joe is formal and respectful to Pip in order to
prevent Pip's social ranking to be affected.