Answer:
Elinor was upset was because there was no factual or formal engagement between Marianne and John Willoughby. She had not seen any confirmation of any kind other than how affectionate they were when around each other.
If there was in fact not any engagement the effect on Marriage could be devastating to her mentality, and her sensibility. She is social with her insightful and quite unrestrained that the prospect of her dealing with no engagement could be extremely detrimental to her well being. Elinor is also not one to gossip or spread rumors so this is quite concerning for her.
