It really struck me as I reread this first section how rigidly Marianne--and her mother--seem to have classified all people into basically two categories: those with and without strong feelings.

For Marianne especially, it seems to be a gulf that cannot be crossed--if you have the strong feelings, you must show them, and if you don't show them, you must not have them. This leads her to errors in judgement.

We are told that John Dashwood 'had not the strong feelings of the rest of the family' and this is certainly true (unless we're talking about a strong feeling for money!)

The narrator also tells us in the first chapter that Elinor "had an excellent heart; her disposition was affectionate, and her feelings were strong: but she knew how to govern them."

We are also told that this ability to govern feelings was "a knowledge which her mother had yet to learn, and which one of her sisters had resolved never to be taught."

Marianne obviously loves Elinor very much, but misjudges her by saying in Ch. 3 "Elinor has not my feelings" and later, after they leave Norland (and Edward) behind, "Elinor, in quitting Norland and Edward, cried not as I did. Even now her self-command is invariable. When is she dejected or melancholy? When does she try to avoid society, or appear restless and dissatisfied in it?"

I think it's important to see Marianne making these serious errors in judgement about her own sister, because she will make similar errors about others, putting them in the category of having no strong feelings because they choose not to put them on display as she does.