Xander's face is what scares her. The look of determination that's so rarely appeared in the past few years is there. The one that makes her feel like she has to listen to him. He wants her to run. He wants her to find her and let her know how she's feeling. Stop her from leaving, if Buffy can.
"Listen, Buffy, I know that you've been through a lot-more, even, than I can possibly imagine. But, stop for a minute. Think it over. She has too. She's been there for you even when you shut us out. She lost Tara this year, Buffy. That alone must have been so much...more than she's had to handle in the past." His jaw is set as he speaks, making Buffy feel as though she should be scared. "But I've looked out for her for as long as I can remember, even more so since we met you. She needs you. Not wants, though maybe that too, sure. But she needs you. And you need her too."
"Xander, I-" Buffy starts, but he puts up a hand.
"Hear me out," he tells her calmly. "Let me know if I'm wrong, but you won't last without each other. If I know anything about you two, I know that much. You love her. Simple as that. It's not that difficult, Buffy. She needs you. You need her. Should be a happy ending, right? No. You're about to let her go. You're ready to let her fly across the world, possibly forever, and you're not even going to try to get her to stay. Am I on track yet?" He paused for a second, not truly expecting an answer. He didn't get one.
"You've done this before, Buffy. You know that you've done this before. And I'm pretty sure this time's different. Don't let her leave. You're an idiot if you do. It's gonna rip you both in half and, right now, you are gonna have to be the one to make sure that doesn't happen."
Buffy looks up at him, hoping that the silent tears tracking their way down her face are enough to convince him that he's right. She lets out a sob; the first one she's released in what seems to have been forever.
"Go," Xander commands. Buffy hugs him tightly, still crying. Then she turns and she goes.