holy tl;dr sorry!
[It's not pain from what happened with Angel or Spike. She's dealt with that and she knows it had to be done. However, that is exactly where her pain and emotions come from. She knows that anywhere else she would eventually be expected to sacrifice everything she loves for the world and a part of her is afraid that through some twist of 'buffy repels happiness' that's going to become true here. And in some way, she's not wrong. She knows that if she gets the chance to go home, she has to take it. She has to take it for the world's sake, for Dawn's sake because she left a giant mess back home and someone is going to have to clean it up. As per usual, that someone is her. It doesn't matter how happy she is here, or if she loves Sam or if he loves her. It's all sacrifice and it all comes at the expense of her happiness. She knows this by now. It's ingrained in her bones the same way fighting vampires is.
His confidence and his words do make her smile a little. She thinks that maybe she could pretend for a while. They make her want to pretend for a while at least.]
I like you being just Sam though. It's kind of a selling point for me.
[She hesitates, but it's a hesitation that's full of possibility rather than one in which she's pondering how to let him down easy. There's a different sort of feel in that space of silence while she lets herself explore the idea of a relationship with Sam.]
I want to try. Probably even more than I want to admit that to myself. [She pauses again. She doesn't have to tell him that it scares her. That's written all over her body language and her emotions. When she speaks, it's very quiet--some habit she got into long ago with Angel and continued throughout her life. When she wants to confess something, it's done in a whisper so soft that one without super hearing would have to strain to hear.]