Pride and Prejudice is an appropriate title for this novel because these are the qualities that Mr. Darcy and Miss Elizabeth Bennet must overcome in order to find happiness with one another: they are intellectual equals who come to realize that they do love the other and would improve each other, and their pride and prejudice could have prevented them from reaching this realization.
Instead, each is willing to recognize how they have been wrong—Elizabeth, as a result of her prejudice and wounded vanity, and Darcy, as a result of his pride—and revise their views of themselves and the world, accordingly. The novel cannot be called Darcy and Elizabeth, or vice versa, because this would make it seem as though the text is simply about these two characters, and it is not. It actually addresses some much larger social issues, like the untenable choices that women often had to make between marrying where they could not love or even respect their partner or ending up an old spinster and financial drain on her family.
It also addresses the double standard that exists concerning marriage and money: as Lizzy points out to her Aunt Gardiner, a person with no money who marries another person with no money is considered to be imprudent, but a person with no money who marries a person with money is considered to be mercenary. How can one win? The novel satirizes these social problems and more and thus is appropriately named after things which might stand in the way of one's happiness but which are surmountable.