Why does Pip lie about his experiences at Satis House in Chapter 9 of Great Expectations by Charles Dickens?

In Stage I of Great Expectations,
Chapter IX is pivotal to the development of Dickens's
bildungsroman, or novel of maturation. For, Pip has been profoundly
affected by his visit to Miss Havisham, and his lying to his family about his visit
reveals the changes taking place within him.  At the beginning of this chapter Pip
significantly narrates,

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If a dread of not being understood be hidden in
the breasts of other young people to anything like the extent to which it used to be
hidden in mine--which I consider probable, as I have no particular reason to suspect
myself of having been a monstrosity--it is the key to many
reservations.

Under the
self-deception that his sister and Uncle Pumblechook "should not understand" the truth,
Pip fabricates a fantastic tale of a black velvet coach, gold plates, and
dazzling flags.  However, later in the evening Pip's conscience bothers him after having
seen the wonder in Joe's eyes.  For, he loves and respects Joe too much to lie to him. 
So, Pip confesses his lie; then he tells Joe that he feels "very miserable" because he
is "common."  To this remark, Joe retorts that lying is no way to "get out of being
common."  Besides, Joe says, in an effort to restore the beloved boy's self-esteem, Pip
is not common; he is "a oncommon scholar."

Indeed, the
experience of visiting Satis House has greatly affected Pip, who now is discontent with
his lifes, anxious about being less than the equal of others and of being
misunderstood.  At the chapter's end, Pip
comments,

That was a memorable day to me, for it
made great changes in me. But it is the same with any life.  Imagine one selected day
struck out of it, and think how different its course would have been.Pause, you who read
this, and think for a moment of the long chain of iron or gold, of thorns or flowers,
that would never have bound you, but for the formation of the first link on one
memorable day.