Why the hell would you? That is not really something you would say to someone without sounding like an insult. You say, "Something's not clicking for me," or "we're just not compatible in a way that's hard to describe." You don't have to say why - I mean, it's just common sense to be polite and have compassion.
Or buy her a nice bottle of perfume. Good stuff, not cheap crap.
Hot chick in the city.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein