I think that Jane uses reason to define her relationship with Mr. Rochester because she was brought up not expressing her emotions; she grew up at Lowood, and Gateshead, where she was not encouraged to express herself. She probably also uses reason because she knows that if she were to use her feelings, her intense love of anf for Mr. Rochester, she would not be in control. Jane is a very independent person; she speaks openly and bluntly with people, which was unconventional in those times. She uses reason to help herself distance herself from Rochester, especially when she ran away in the night. If she did not use reason, she would have stayed, and the rest of her story would have been different. Reason to Jane is her being in control and it's the only thing she really knows.