Well, the line is a bit of a non sequiter, when you take into account the dialogue preceeding it:

(Darcy) "In a country neighborhood, you move in a very confined and unvarying society."
(Lizzy) "But people themselves alter so much, that there is something new to be observed in them for ever."

As far as I can tell, she's just trying to defend country living, without really understanding what the argument is.