When Lizzie first meets Mr. Darcy she is intrigued by him. As she tries to engage him, she finds him cool and aloof. When he tells Mr. Bingley, in a conversation overheard by Lizzie, that "she is tolerable; but not handsome enough to tempt me;", in typical fashion and pretending no insult, Lizzie begins to ridicule Mr. Darcy and his ways.
When she meets the charming Mr. Wickham, she is instantly taken with him. After seeing Darcy's reaction to Wickham, Wickham goes on to explain he grew up with Darcy and was cheated out of his inheritance by the man. Lizzie instantly uses this information to cement her already bad feelings toward Darcy.
The irony comes as later events unfold and Lizzie discovers that Wickham has foolishly spent all his inheritance. He even attempted to convince Darcy's sister to run away and marry him in order to get her money. Lizzie realizes then she accepted Wickham's version with no question. To make matters worse, her sister runs off with Wickham and he makes it known he can be bribed into marrying her. When the two newlyweds arrive at the Bennets' home however, Lizzie finds out Darcy was the one who took care of all the arrangements, including purchasing a commission in the army for Wickham.