Austen created Elizabeth's image to be that of a very strong-willed, independent, intelligent woman. She is so strong-willed that she is actually a bit rebellious. She even thinks very highly of her intelligence and her ability to judge. Hence, when Darcy delivers a proposal saying that it was against his better judgement to ask her to marry him and even a degradation of his pride, it is no surprise that she refuses him, due to her independent, strong-willed, self-admiring spirit,.
We first see Elizabeth's independent, strong-willed, witty, intelligent nature when she is snubbed by Darcy at the ball. When Darcy says that Elizabeth is "tolerable; but not handsome enough to tempt me(it)," instead of being put out by the insult, Elizabeth finds it humorous. Someone who was more dependent on others for his/her happiness and less self-assured might have let Darcy's comment become hurtful. However, Austen describes Elizabeth as later telling the story to her friends "with great spirit," because she has a "lively, playful disposition, which delight[s] in any thing ridiculous" (Ch. 3).
Later, we see Elizabeth's independent nature in her dealings with Darcy, specifically when she is staying at Netherfield with her ill sister. She very frequently makes impertinent remarks to Darcy. For example, when he asks her if Miss Bingley's piano playing makes her want to dance a reel, or jig, her response is:
You wanted me, I know, to say "Yes," that you might have the pleasure of despising my taste; but I always delight in overthrowing those kinds of schemes. (Ch. 10)
Elizabeth is again being the wit here by overthrowing what she thinks is another opportunity for Darcy to insult her, thereby again demonstrating her independent and strong-willed mind.
Hence, since Elizabeth is so very independent, strong-willed, and thinks so very highly of herself, it is absolutely no surprise that she rejects Darcy's proposal when in his proposal he says that she's beneath him.
Austen makes Elizabeth a 'non-victorian' caharcter; usually, when a woman gets proposed to they would accept as they might not get a secon proposal and marriage was very important in the victorian era. her refusal to Collins proposal tells the audience what kind of character she is; stubborn and she does not wish to marry a man with wealth and inheritence, but a man with a good personality. this is whats she says earlier on in the book.