While I think that was very much the case with Eddie, I got the impression it was a slower process for Odetta/Susannah. I think it happened quickly but I don't really think it was Love at first sight for her. I also get the impression later that he loves her more than she loves him*, although they were both very much in love. And I do think their circumstances; their need for each other, were a large factor of that. Which isn't to say it wasn't Ka of course as all things serve Ka.
*I don't intend that last statement to be disparaging of Susannah's character. She could be pretty annoying but she was a great well rounded** character.
**Insert lady-lump 'twins' gag here. No, I honestly didn't mean it like that...