The major themes of both William Shakespeare's Romeo and Juliet and Jane Austen's Pride and Prejudice revolve around love. In both cases, family and social circumstances provide obstacles to love, but they differ in that Austen offers a happy ending and Shakespeare an unhappy one, and in Shakespeare's case the obstacles to love appear purely external and in Austen's internalized.
In Romeo and Juliet, the two young lovers belong to feuding families and decide to elope. Their elopement scheme goes horribly wrong through accidents of miscommunication, and both young lovers commit suicide. Although the play can be read as an example of true love, the unhappy ending also suggests another reading, in which the characters are seen as infatuated with each other and overly impatient. The illicit aspect to the relationship also has to do with Juliet being an impressionable 13-year old and Romeo perhaps in his twenties, and the elopement almost seeming a form of sexual exploitation of a very young girl by an older man.
The couple most resembling Romeo and Juliet in Austen's Pride and Prejudice is not the Darcy/Elizabeth pair, who are both fully adult and do not marry until they have the consent of Elizabeth's family, but rather Lydia and Wickham, a pairing between an older man and young girl involving an illicit elopement and an unhappy marriage. As in Shakespeare's play, these two are emotional and impulsive, have not internalized social conventions, and tend to act on impulse rather than out of rational thought. Like Romeo, Wickham is fickle in his affections. One rather wonders if the two lovers of Shakespeare, had they lived, would have settled into the sort of bad marriage that evolves between Lydia and Wickham.