Beyond this, I think Marianne and Mrs. Jennings conclusions and presumptions are drawn from different sources and for different reasons.

Where Marianne imagines all the world to have her feelings, she imagines how she would feel and act in a situation, and believes others would too. We know that a Marianne in love feels herself promised to the one she loves. So, believing Elinor to be in love, she cannot imagine that Elinor would be uncertain of her own future bliss. Marianne is, in the quoted passage, offering her sister support and encouragement in what she believes to be Elinor's iminent engagement.

Mrs. Jennings, on the other hand, draws conclusions from a desire to have something to talk about and to plan. She is not, like Marianne, painting people's actions and desires in light of what her own would be, but rather, she is colouring people's behaviours and actions to create the most exciting and dramatic story possible. Mrs. Jennings is, if a kind hearted person, also a terrible gossip. She likes to make guesses about situations, create romances, and tease people for the sake of her own entertainment.