Pip tells Biddy he has “particular reasons for wanting to be a gentleman” in chapter 17 (enotes etext p. 29). He then makes the following “lunatic confession” to her.
The beautiful young lady at Miss Havisham's, and she's more beautiful than anybody ever was, and I admire her dreadfully, and I want to be a gentleman on her account. (ch 17, p. 90)
When Pip first meets Estella, he is already smitten with her. She offends him by saying that the soles of his boots are too thick and his hands are coarse, and he uses the wrong names for the cards when they play cards.
Once alone, Pip takes the opportunity to look at his “coarse hands” and “common boots.”
My opinion of those accessories was not favourable. They had never troubled me before, but they troubled me now, as vulgar appendages. (ch 8, enotes etext pdf p. 44)
He now feels as if he is not good enough, not just for Estella but in general. This event, and his visits to Miss Havisham’s house, essentially scars him. He begins to be ashamed of himself. He knows that Estella would not approve of Joe—a “mere blacksmith” and therefore would not approve of his future either (ch 9, p. 50).
Pip begins his career as a blacksmith’s apprentice, but he continues to pine for Estella. He is ashamed not only of himself, but of Joe, whom he wants to make “less ignorant and common … and less open to Estella's reproach” (ch 15, p. 77). He begins to be “ashamed of home” (ch 14, p. 75). He measures everything in terms of how she would see it.
Miss Havisham does not just destroy Estella with her games, she destroys Pip too. Pip’s expectations of being a gentleman, given to him so generously by Magwitch, result in disaster. His desire to make himself better for Estella is unrealistic, as he realizes when he tells Biddy, but he cannot get her out of his mind.