Why does Mr Collins propose to Charlotte only three days after Elizabeth in Pride and Prejudice?

Mr. Collins proposes to Charlotte because first of all he needs a wife. Lady Catherine advised him to marry, and he lives to please her. Also, Charlotte leads him on, she drops hints and wants him to propose to her because she thinks she can live a good life with him since she is not romantic. Mr. Collins didn't really love Lizzy, he just wanted to bring a wife back to Hertfordshire with him.