As the character of Elizabeth Bennet unveils before the reader's eye, there are specific traits which stand out quite strongly.
First, there is Elizabeth's independent thinking. She does not abide by the social expectations of "marrying well" in order to fit in society. She is determined to find a man whom she will love, regardless of financial status. She also has no hurry to marry, in contrast to Charlotte Lucas, for example. All these qualities denote a young woman who was ahead of her peers in terms of self-confidence, self-assurance, and self-respect.
This being said, we can easily conclude that Elizabeth's image, created by Jane Austen, certainly matches the fact that she would refuse to marry Mr. Darcy. When Mr. Darcy proposed to Elizabeth he did it in the most awkward way. He even pointed out that she belonged him, socially speaking, and that marrying her would be doing a disservice to his family.
Any woman who would be desperate enough to marry, especially to marry an aristocrat like Mr. Darcy, would have taken that as an honor. Yet, Elizabeth could not care less about rank or position: Her family, her dignity, and her sister's happiness came before any desire of her own. Therefore, her refusal to marry Mr. Darcy is not only foreseen, but also expected.