Certain it was that she was to come; and that Highbury, instead of welcoming that perfect novelty which had been so long promised it -- Mr. Frank Churchill -- must put up for the present with Jane Fairfax, who could bring only the freshness of a two years absence.

Emma was sorry to have to pay civilities to a person she did not like through three long months! to be always doing more than she wished, and less than she ought! Why she did not like Jane Fairfax might be a difficult question to answer; Mr. Knightley had once told her it was because she saw in her the really accomplished young woman, which she wanted to be thought herself; and though the accusation had been eagerly refuted at the time, there were moments of self-examination in which her conscience could not quite acquit her. But "she could never get acquainted with her: she did not know how it was, but there was such coldness and reserve; such apparent indifference whether she pleased or not; and then, her aunt was such an eternal talker! -- and she was made such a fuss with by every body! -- and it had been always imagined that they were to be so intimate -- because their ages were the same, every body had supposed they must be so fond of each other." These were her reasons; she had no better.

It was a dislike so little just – (Chapter 20)

Perhaps Jane is a reserved person. Emma has not been her friend in the past so even if they were at the beginning of a beautiful relationship I don’t see any reason Jane must open up her life or views to Emma’s examination just because Emma wants to satisfy her curiosity. Most friendships have to grow before trust can be gained and I cannot think of a reason Jane should feel impelled to explain her feelings about people Emma has never even met.

How do you know Emma has never done anything to deserve Jane’s reserve towards her? Per Chapter 20, Emma’s dislike of Jane in the past was “little just” and do you suppose Jane was too stupid to feel this from Emma? Since you say Jane can have no reason of merit for her reserve towards Emma after a two year absence do you also fault Emma for disliking Jane after a two year absence?