She's only going to live nine years, she doesn't need that lung.
I assume the Ocampans must mate for life, since they don't live long enough to get sick of each other.. and she was probably at a loss as to how to break up with someone and what was required. I bet she took all her cues from the soap operas she watched in Future's End!
Kes has total recall on everything.
Every second of every day with the same man for 6 or 7 years?
Humans forget 90 percent of the unimportant stuff, and we still want to kill our spouses before the first year has rung out.
She has the human equivalent of centuries worth of "life with Neelix" after only a few months.