regarding the reason for Emma's 'dislike' of Jane Fairfax. I believe had Mr. Knightley not been the one who praised Jane Fairfax so openly, Emma would not have developed such 'dislike' in Jane.

In Chapter 48 “Till now that she was threatened with its loss, Emma had never known how much of her happiness depended on being first with Mr. Knightley, first in interest and affection. Satisfied that it was so, and feeling it her due, she had enjoyed it without reflection; and only in the dread of being supplanted, found how inexpressibly important it had been. Long, very long, she felt she had been first; for, having no female connexions of his own, there had been only Isabella whose claims could be compared with hers, and she had always known exactly how far he loved and esteemed Isabella. She had herself been first with him for many years past. She had not deserved it; she had often been negligent or perverse, slighting his advice, or even wilfully opposing him, insensible of half his merits, and quarrelling with him because he would not acknowledge her false and insolent estimate of her own -- but still, from family attachment and habit, and thorough excellence of mind, he had loved her, and watched over her from a girl, with an endeavour to improve her, and an anxiety for her doing right, which no other creature had at all shared. In spite of all her faults, she knew she was dear to him; might she not say, very dear?”

Emma was never aware of how important of being first in Mr. Knightley’s life until here, but her desire to be his first had always been there, likely since she was a very young girl. Even though Jane Fairfax had never supplanted Emma’s place in Mr. Knightley’s heart, the fact that he did not praise Emma often – “I could not think about you so much without doating on you, faults and all; and by dint of fancying so many errors, have been in love with you ever since you were thirteen at least.” Chapter 53 – and his good opinions on Jane were so obvious it must have created some uneasiness or even subtle bitterness in her. Had it been someone else who admired Jane Fairfax so openly I believe the effect on Emma would have been minimal.

It’s interesting (and endearing) that the misunderstandings, as well as parallels, in Emma, the book, run so deep. Readers are revealed that it was Mr. Knightley’s jealousy for Frank Churchill that created his dislike of Frank, but I have always believed that the root of Emma’s dislike of Jane Fairfax stamped from her jealousy for Jane Fairfax over Mr. Knightley’s favorable opinions. I am so glad that Miss Austen had squared away all misunderstandings between these two love birds at the end of the book, they were such a match that was meant to be from the beginning of time! :-)