What makes the humour in Jane Austen's Pride and Prejudice.
It is a truth universally acknowledged, that a single man in possession of a good fortune, must be in want of a wife.
This verbal irony is humorous because it is not a universal truth that all single wealthy men want wives, yet it was widely known in Austen's time that mothers of five single daughters acted as though it were true.
Irony is humorous because of the witty figures of speech that comprise verbal irony and the amusingly unexpected twists of fate that comprise situational irony (amusingly unexpected in a comedy, at any rate). Irony occurs when words have a figurative twist to them so they mean something other than what they seem to mean or when situations prove to be something other than what is expected.
Austen develops situational irony in a number of instances. One thing about Austen's ironic situations is that sadness of varying degrees accompanies the ironically humorous situations. For instance, we are presented with an ironically humorous situation when Mary insists on singing loudly and badly at the Sir William’s gathering but there is also sadness attached to it. In the first place, Mary is humiliated because her talent is not equal to her vanity:
Mary had neither genius nor taste; and though vanity had given her application, it had given her likewise a pedantic air and conceited manner, which would have injured a higher degree of excellence than she had reached.
In the second, Darcy adds Mary’s behavior to the long and growing list of reasons he presents to Bingley to dissuade him--successfully--from proposing to Jane. Thus, it is through the addition of sorrow to humorously ironic situations that Austen develops her themes. Similarly, Austen employs humorous verbal irony to develop her plot and her characters.