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,
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.
Basically, Pip does not want to tell about what happened to him at Satis House because he is stubborn and the people are pushing him too hard to talk about what happened. Here is a funny quote that kind of tells you why Pip doesn't want to tell:
Whitewash on the forehead hardens the brain into a state of obstinacy perhaps.
In this line, he is referring to how he has been bullied into talking. He has had his head pushed against the wall by his sister, for example.
So Pip is just feeling stubborn. He does not like the ways in which Mrs. Joe and Pumblechook are pushing him to tell everything that went on.