In Chapter 57 of Great Expectations, why does Joe's treatment of Pip change as Pip gets well?

After the ordeal of saving Miss Havisham and getting
burned and the long and the enervating attempts to save Magwitch from hanging, and his
long vigil with poor Magwitch who dies from his injuries, Pip, heavily in debt, returns
to his apartment and falls ill.

As he lies in bed wavering
between delirium and consciousness, he, at last, perceives the face of his dear friend,
Joe, and asks, "Is it Joe?" It is the old Joe who replies, "Which it air, old
chap."

Guiltily, Pip expresses his shame and appreciation
that Joe has come to nurse him. But, Joe explains that Biddy has encouraged him to waste
no time in attending to Pip after he is notified of Pip's grave illness. As Pip gains
some strength, Joe takes him outdoors. Pip recalls,

More
composure came to me after a while, and we talked as we used to talk, lying on the grass
at the old Battery. There was no change whatever in Joe. Exactly what he had been in my
eyes then, he was in my eyes still; just as simply faithful, just as simply
right.

However, as Pip gains his strength and returns to
himself, Joe begins to withdraw the old familiarity; one day Pip remarks to Joe that he
soon can walk by himself. And, Joe replies,

Do not overdo
it, Pip...but I shall be happy fur to see you able,
sir."

When Joe says "sir," Pip
is disconcerted. He notes,

Joe became a little less easy
with me. In my weakness and entire dependence on him, the dear fellow had fallen into
the old tone, and called me by the old names, the dear “old Pip, old chap,” that now
were music in my ears. I too had fallen into the old ways, only happy and thankful that
he let me. But, imperceptibly, though I held by them fast, Joe's hold upon them began to
slacken; and whereas I wondered at this, at first, I soon began to understand that the
cause of it was in me, and that the fault of it was all
mine.

Pip realizes that while he has been weak and ill, he
has been again the little boy that Joe so loved. However, now that he has regained his
strength, Pip is again the gentleman who shunned Joe on his visit to London. Seeing Pip
as an adult again, thus, makes Joe uncomfortable and awkward around Pip. So Pip
apologizes to him,

"We have had a time together, Joe, that
I can never forget. There were days once, I know, that I did for a while forget; but I
never shall forget these.”

Nevertheless, Joe departs,
generously having paid Pip's debts, and having written to Pip that he does not wish to
"intrude." Reading his letter, Pip knows that he must return to the forge, penitent as
the Prodigal Son, and beg his forgiveness.