The buffyverse and all its characters belong to some very nice people, who also have very nice lawyers. I am making no money of this and am making no claims to these characters.
She knew when she said it he wouldn't believe it. Knew it and said it anyway. Why? Because sometimes, there's no time left to make it true, sometimes just saying it has to be enough, because if you don't you'll never know if you should have said it.
And when she remembers the look in his eyes; hope and pity and a thousand other things besides, she knows she'll never regret it, that saying that to him will go on that big list in the her head of good decisions.
Which is not to say that this memory doesn't hurt. It does. All the time. It's there behind her eyes if you look. The immortal knows this. Knows that's why she's dating the undead again; in times of change we cling to what we know. Dating the undead is all Buffy's had. That and Captain Cardboard.
So she lives in Rome, in its beauty in its splendour. So she dates the undead, which, lets face it, even she knows is a euphemism for: "fucks poor Spike substitute."
Those things are facts, open to the public record.
She knows he's back. The nerdster could never keep something this big, this romantic from his hero(ine).
And she's been waiting.
When she felt the tingle of slayer sense and maybe something else in the night club she thought he'd come round.
God knows a slayer couldn't run to the ungrateful undead. She had to wait for him to come to her. She'd put on a show, wanted to make him jealous. Maybe she did. She doesn't know.
And now she hears that there's some sort of end of the world thing happening in la-la and she's not been asked to the party. She can feel this is the real deal. She can tell that he won't be getting out of this one alive.
She knew when she told him he wouldn't believe it. She thought when she heard it was back she'd get the chance to show him. She doesn't want to be wrong.
Sometimes there's no time left to make something true. Sometimes you have to do something, because you won't be able to live with yourself if you don't. A flight from Rome to LA only takes a few hours.
Prehaps a few hours is enough time to make something true.