Reading through the posts on Marianne's devaluing of Brandon, it struck me that she's very reactive. She tends to react against (i.e. take an opposing view as) people like Mrs. Jennings and, at times, Elinor, and to react with (i.e. take the same opinion as) people like Willougby and, at times, her mother. For the ones who elicit stronger reactions in her (Mrs. Jennings, Willougby), she seems to be reacting out of sheer prejudice (against or for) the person, rather than actually considering the merit of what they're saying. I think that this is another example of her immaturity, and a sign that perhaps her opinions aren't as set as she professes. She's got a lot of room for growth here!