I rather liked Jaffen. I don't think she was in love with Kashyk at all but she was quite likely in lust/attracted to him. Playing the role of seducing him to believe in her would have rubbed off on her as well.
You do remember that they were living together?
it's quite likely that she was programmed to make roots and form connections, so that she would find it more impossible to look for another job on a different part of the planet.
If kathryn was compelled to find a live in boyfriend and couldn't help herself, then maybe Jaffin couldn't have made the cut ordinarily?
Sure she did ask him to join Voyagers crew, but that's the secret gamble of taking home drunk people from night clubs, when they wake up already covered in your issue, they have to justify decisions they can't remember and assume that you would have made the cut if they were sober, which allows you at least one more attempt at sex in the morning before they decided to flee or stay, hangover permitting.
The Prime Directive.
It's only okay to take home "strangers" who are as equally drunk as you are.