Why does Charlotte believe that Mr Darcy must be in love with Elizabeth?

In Chapter 32 of Pride and Prejudice, Charlotte returns home from a walk to find Mr Darcy visiting with Elizabeth. This causes her to say:

What can be the meaning of this? My Dear Eliza, he must be in love with you, or he would never have called on us in this familiar way.

On one level, Charlotte is alluding to the difference in status between herself and Elizabeth and Mr Darcy. As a member of the upper class, it certainly is unusual that a man of such status would call on Elizabeth, his social inferior, seemingly without reason. This leads her to believe that Darcy must have a personal reason for this visit; namely, that he is in love with Elizabeth.

On a deeper level, however, Elizabeth's reluctance to accept Charlotte's conclusion demonstrates the extent of her prejudice towards Darcy. This is caused, primarily, by her belief that Darcy is the devil in Mr Wickham's tale of woe and also that Darcy has actively discouraged a romance between Bingley and her sister, Jane. 

For Darcy, however, this is an important turning point in overcoming his pride. He is in love with Elizabeth, but he acutely feels the difference in their social statuses. This chapter sets the scene for his marriage proposal, in which Darcy's pride and Elizabeth's prejudice will go head to head.