Interestingly, Austen originally planned to call this novel First Impressions, and there is a sense in which this still has a very legitimate place as the novel's title, as so much of what happens is based on the first impressions of Elizabeth and others towards the characters that enter their lives. Her first impressions of Darcy lead her to dismiss him as proud and arrogant whereas her first impressions of Wickham lead her and other people to think him the most wonderful individual ever. Note what Elizabeth feels about Wickham at the end of Chapter 16, when she has first met him:
Whatever he said, was said well; and whatever he did, done gracefully. Elizabeth went away with her head full of him. She could think of nothing but Mr. Wickham, and of what he had told her, all the way home...
First assumptions are therefore crucial in this novel as it helps to create so much of the conflict and also exposes the way that humans are so prone to trust on first impressions, even before somebody else is truly known. Elizabeth's first impressions, once they are established, are very hard to break down and correct.
Jane Austen's theme of "first assumptions," or better phrased as "first impressions," can easily be seen in Elizabeth's first reactions towards both Darcy and Wickham.
Elizabeth formed her first impressions of Darcy at the Meryton ball. Like everyone else present at the ball, Elizabeth soon saw Darcy to be "proud, to be above his company, and above being pleased" (Ch. 3). Elizabeth's ill opinion of Darcy is intensified when Darcy delivers his famous snub, saying that he will not dance with Elizabeth because "she is tolerable; but not handsome enough to tempt [him]" (Ch. 3). However, her first impressions are further solidified when she meets Wickham. While she thinks Darcy to be proud and conceited, Whickham tells her a story about Darcy that surprises her, making her believe that Darcy is the worst man she has ever met. Not only that, the only reason why she believes Wickham in the first place is because her first impressions of Wickham are that he was a friendly, polite, handsome, and well-mannered man. Unlike Darcy, Wickham is openly conversational and friendly, which makes Elizabeth believe at first that he is the better, more trustworthy man.
However, regardless of her first impressions, Elizabeth soon learns that she had very prejudicially misjudged both Wickham and Darcy equally.