(This is a difficult thread to jump into, because there are at least thirteen four posts that relate to the little bit I want to add.)

Mr Knightley retrospectively deems himself to have been in love for many years, but I favour fascination as a cool-headed and precise term for what his feelings would've been at the time. It sits within reach of love, and of anxiety and curiosity, which occur in the speech Nina RG quoted from chapter 5.

Emma's own reaction to the "...in love with you ever since you were thirteen at least" remark in chapter 53 is also a clue to its tone. She refers (playfully, in my opinion) to his not being to fall in love with baby Anna Weston in another thirteen years, and (wryly, in my opinion) to her own past amiable behaviour.