During the time that Great Expectations is set, Britain was going through rapid social, economic, and political changes. In the early Victorian era, Britain was the world's leading industrial nation, with an enormous empire that stretched right around the globe. On the domestic front, a whole new generation of self-made men drove the nation's prosperity, using their newfound wealth to achieve a degree of social prominence previously denied to them.
Yet although Britain was changing, this was still a deeply class-conscious society, as we can see in a number of places in Great Expectations. Abel Magwitch, for example, though rising up in the world to become a successful, wealthy businessman, will always be tainted by his dark past as a convict. The shocking revelation of his being Estella's father ruins his daughter's chances of a good marriage.
Then there's Pip. Although Pip's temporarily able to lead the life of a wealthy young gentleman about town due to Magwitch's generous bequest, he's still never fully able to escape his humble background as a "common laboring boy" (as Estella cruelly calls him) growing up on the bleak Romney Marshes. Both Estella and Pip have been undone by the prevailing social snobbery, and it's this common outsider status that ultimately brings them together.