I also read the introduction to the Norton Critical Edition. Mrs. Dashwood and Marianne are of a like mind (to wish was to hope, and to hope was to expect) and Margaret appeared to follow their path. Prior to the death of Mr. Dashwood, they might have had a sheltered existence. So why was Elinor more rational and doubting? Naivety is often destroyed by bitter experience, although some people can learn from the mistakes of others. We know of no bitter experience in Elinor's life prior to the start of the novel so I'm wondering if she, as the eldest daughter, had more responsibility thrust upon her. Perhaps she was old enough to listen to the advice or experiences of her father or other relatives.