In Great Expectations, Mr. Jaggers advises Pip, "Take nothing on appearances." Certainly, the Pip-Estella relationship is an example of the Appearances vs. Reality theme that prevails thoughout Charles Dickens's classic novel.
From the first meeting of Pip with Estella, Pip falls victim to believing in appearances. The beautiful, haughty girl whose name means "star" is elevated in Pip's esteem simply because she lives with the rich Miss Havisham and is dressed in lovely clothes and speaks in a deprecating way to him, calling him "common." Immediately, because this vision of superior loveliness who speaks properly has termed him "common," Pip experiences a humiliation. But, despite her cruel ways, Pip falls hopelessly in love with the beautiful Estella, perhaps even because she is unattainable. He perpetuates his delusions by hoping that if he becomes a gentleman, Estella will accept him as an equal and requite his love.
Of course, the truth is that from the beginning, Pip's birth has more legitimacy than that of Estella's. For, his parents were married and, albeit poor, they were certainly not criminals as are the parents of Estella, whose birth came out of the streets of London.
In addition to the theme of Appearance vs. Reality, the relationship of Pip and Estella also points to a salient theme in the works of Dickens: Class Stratification. The theme of social class is central to Great Expectations as it acts as extends into the other themes such as the Appearance theme. Pip's angst over being "common," as Estella has labeled him, is his driving force to become a gentleman and entertain the "great expectations" of having bettered himself sufficiently so that he will become worthy of Estella. But, of course the class structure is a false one in Great Expectations, thus paralleling the Appearance vs. Reality theme, as Pip later learns; rather, it is what one is as a person that is truly of value. Estella, for all her beauty and daintiness is but common in her heart; she is incapable of noble thoughts and acts while Joe, the humble blacksmith is truly a good and noble man.
To me, one of the major themes of the novel is the idea that suppressed emotions can make a person into something of a cripple. We see this most clearly in the character of Miss Havisham.
But we also see this theme some in the relationship between Pip and Estella. Estella is something of a cripple herself because she cannot act according to her true feelings. Miss Havisham has raised her in such a way that she can't get past hating men. But this is not how she truly feels. We see this in the fact that she keeps trying to spare Pip from harm even as she is harming him.
The relationship between the two of them is a very dysfunctional one, with Pip loving her and her trying to hurt him. Dickens is trying to show us (through this relationship) how much suppressed emotions can hurt people.