When I said that, I was thinking more of:

Vivacious: “full of high spirits” and “vitality”

Fun-loving: taking pleasure in “enjoyment” and enjoyable pastimes

(Definitions from the Collins Concise Dictionary, 1985 and Chambers Dictionary, 1994)

To me, the above definitions fit Marianne well. I was not referring to Marianne being fun-loving in the manner of Lydia, whereby fun-loving implies flirting and frivolousness, but in a gentler, innocent way. Marianne is “…eager in every thing; her sorrows, her joys, could have no moderation…every thing but prudent” (Ch. 1).

“…[she] embodies both the refinement of modish sensibility and the dangers arising from hypersensitivity…Marianne is a Romantic; her enjoyment of the natural world recalls Austen’s contemporaries, the poets Coleridge, Keats and Shelley. She is suspicious of the formality that her social group tends to expect. Early in the novel she expresses an aversion to everything that might be termed ‘illiberal’, and it is clear that by this she means anything that compromises her Romantic enthusiasms” (S&S, Collector’s Library Edition, Afterword).

I would argue that Marianne is full of vitality and high spirits, in short, vivaciousness. But then different words conjure up varying images for people; when I describe Marianne as vivacious and fun-loving, I mean this in a very gentle, soft way. At the beginning of the story, Marianne’s feelings are at an extreme; she is either in the depths of despair or climbing the heights of happiness. She may learn to regulate her sensibility and emotions, or she may not. Who knows what will happen to her? ;-)