The principal (main) character in Great Expectations is Phillip Pirip, also known as Pip (a pip is an apple seed, known in Spanish as pepas, or pepitas).
The importance of Pip's emotions is that they are being described and analyzed from the point of view of a young boy growing into maturity, and then becoming a society gentleman despite having come from poverty. This is an essential point for a Bildungsroman, or "formative novel". Great Expectations specifically details the events leading up to Pip becoming an adult.
Given that Pip is poor, an orphan, and unjustly treated as a child, the sad and infuriating emotions that he experienced are going to be eventually weighed against the emotions of a more mature Pip who, as an adult, carries the traumas of his childhood and tries to deflect them. Yet, deep inside, those emotions will always be there, creating an invisible barrier for Pip to comfortably accept his past for what it was.
Emotions define how far Pip is willing to go to move away from his origins and impress Estella; again, the memories of his childhood continue to press against him while he continuously does his best to forget all about it.
I went along, on all I had seen, and deeply revolving that I was a common labouring-boy; that my hands were coarse; that my boots were thick; that I had fallen into a despicable habit of calling knaves Jacks; that I was much more ignorant than I had considered myself last night, and generally that I was in a low-lived bad way.
When Pip is asked to go to Miss Havisham's home, he sees for the first time the difference between lavish living and the type of life he had. This, coupled with the inheritance that he gets, and his firm believe that Miss Havisham is his benefactor made him look down upon his family, and give him a false feeling of being born a dandy, or a gentleman. Money does get over the top for Pip, and that is where his "great expectations" lay; on him succeeding in becoming a wealthy man who is entirely different from the abused little poor boy that he was.