Forgive me for saying this, but I think Charlotte buys much more into the idea of marrying for economic reasons alone than Lizzy and Jane do. They have the romantic view of marriage. Charlotte, at 27, is practically an old maid, and she is not as pretty as they are. She seems a rather serious person too, not clever and witty, which are qualities that attract men, also. She doesn't strike me as being the flirtatious type either. Because she is often a bystander, an observer or "studier" of others, like Lizzy, she sees the interactions between people and is able to assess how one's behavior might look to another. I think Charlotte is less emotional, also, than her friend Elizabeth. Lizzy has been wounded by a rude comment by Darcy, but Charlotte has not, and sees his behavior in a wider perspective.