... begins:-

This was the season of happiness to Marianne.

This sentence always seems to me like a red alert that Marianne's season of happiness will not sustain through the rest of the novel. Besides, there are still another forty chapters to go, so unless the main element of the narrative proves to be about Elinor rather than Marianne, something is sure to occur to puncture Marianne's bubble of happiness, otherwise where is the plot to come from?

Willoughby can certainly be criticised on many points, but like Karen G I don't find anything in chapters 9-15 to make me think he is irredeemably a Bad Thing. It is clear that he is no Mr Knightley or Edmund Bertram, but the early descriptions of him have much in common with the way the young Captain Wentworth is described in "Persuasion" chapter 4:-

...a remarkably fine young man, with a great deal of intelligence, spirit and brilliancy..

rapidly and deeply in love... A short period of exquisite felicity..

..nothing but himself to recommend him, and no hopes of attaining affluence but in the chances of a most uncertain profession... ..had no fortune. He had been lucky ... but spending freely, what had come freely, had realized nothing...full of life and ardour,

confidence, powerful in its own warmth, ...the wit which often expressed it... dangerous character... imprudence... brilliant... headstrong.

Willoughby is not Wentworth, but I know from Persuasion that JA can create a vivid, ardent, spendthrift, somewhat dangerous young man and turn him into a Hero.

So I do not despair of Willoughby or of Marianne's ultimate felicity at this early stage of the novel. The manner of Willoughby's departure looks very odd, to be sure. But Brandon also also departs in a hurry without clear explanation, and when Edward returns to Barton his behaviour is strangely altered. All three men appear to have something to hide....