I have always enjoyed the witty opening of Pride and Prejudice.  From the first sentence, the book pronounces both its subject and its tone.

The very first line in the book is a joke about marriage.

IT IS A TRUTH universally acknowledged that a single man in possession of a good fortune must be in want of a wife. (ch 1, p. 4).

With this sentence, Austen sets the tone for the book—biting but witty—and the subject—marriage.

The sentence introduces the elements of the relationship between husband and wife, especially in terms of spending money.  The conversation between Mr. and Mrs. Bennet is also quite humorous, poking fun at the marital relationship.

Mr. Bennet made no answer.

“Do you not want to know who has taken it?” cried his wife, impatiently.

“You want to tell me, and I have no objection to hearing it.”

This was invitation enough. (ch 1, p. 4).

The stereotype presented in the first line, that women spend their husbands money, is further developed to the idea that women talk too much, and care about money and status, and men just humor them.

Thus, from the beginning we know that Pride and Prejudice will be a book about marriage, relationships, and finding humor in them!