Elizabeth turned down Mr. Collins's proposal. What reason did Elizabeth give for her refusal?

Elizabeth wants to marry for love and not convenience. She really does not like Mr. Collins and is rather turned off by his desperation, 

`your hope is rather an extraordinary one after my declaration. I do assure you that I am not one of those young ladies (if such young ladies there are) who are so daring as to risk their happiness on the chance of being asked a second time. I am perfectly serious in my refusal.'