In this passage from “On Making an Agreeable Marriage,” what surprises Jane Austen about her niece’s feelings for Mr. J. P.?

"On Making an Agreeable Marriage" is a title that has been appended to some of Austen's writings to her niece Fanny on the subject of the latter's marital affairs. In a letter dated November 18, 1814, Austen says that she was very surprised at the change in her niece's feelings because she had at first thought that Fanny was "really very much in love." However, now it seems that there is no such attachment on Fanny's part, which Austen finds strange because it is as if "being secure of him had made you indifferent." Austen assesses that Fanny had never been in love but had simply had the "acuteness" and "taste" to be fond of her lover. Now, it is very strange to Austen that the change in her niece's feelings has been "so great," even though her beloved has not changed at all, except to become "more evidently and uniformly devoted to you." This seems, to Austen, to be a strange and unexpected reaction, indicating as it does that as the lover's devotion increases, Fanny's love for him declines. Naturally, this is the opposite of what might be expected, and Austen then proceeds to wonder whether, because this was the first young man who had paid attention to Fanny, Fanny believed herself to be in love with him for a reason that had nothing to do with the young man himself.
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