By having her protagonist declare fairly early on that she will only be happy if she marries for love, and in the end she holds out and does just that, it proves that romantic love trumps all of the other motivations. Elizabeth and Darcy have the most romantic of the loves in the novel because they have been tested and still manage to perserver through all of the misunderstandings and interferences. Elizabeth's refusal to marry Collins is evidence of true love being her priority. Charlotte's "marriage of convenience" but not love shows the opposite of Austen's theme. Lydia and Wickham do not have a romantic love, they are an infatuated lust, at best.
What makes the end of the novel more satisfying is the ironic point Austen makes. Elizabeth, by holding out for true, romantic love, actually gets the whole package -- love and financial security with Darcy.
'Romantic love' is the central theme which unites all the incidents and the characters in "Pride and Prejudice." But there is nothing 'romantic' about Jane Austen's treatment of 'romantic love' in the novel. 'Romantic love' is checked and controlled by the incomes and financial freedom of the partners involved. In this manner Jane Austen is able to blend 'romance' and 'realism.' For example, Lydia and Wickham who elope 'romantically' have to be rescued by the generosity of Darcy before they are married.
The restraining power of money on 'romantic love' is spelt out in the thematic statement found in Ch.27, "Where does discretion end, and avarice begin?", when Elizabeth replies to her anunt's query concerning Miss King the latest lover of Wickham. Her aunt is relieved to know that Elizabeth is not in love with Wickham who has virtually no income at all and is only employed temporarily in the Militia.