.
Harmony seemed to be repeating Angel's speech: Spike has a pattern with women and he doesn't change. She says Spike has idealized Buffy and now he is not happy with the real deal. Yeah, in the mind walking, Buffy saw how Spike sees her – perfect, even glowing (who doesn't see the loved one like that?), but she also felt that his love was real and strong. Besides, he himself already said that he saw her best and her worst, so he knows with whom he is dealing. And Vicky, talking to Buffy, repeated word by word Spike's speech, saying she “always want what she can't have” - otherwise she gets bored.
I haven't read this entire issue yet, and maybe this question is answered there, so please bear with me...
"Spike has a pattern with women, and he doesn't change." he stayed faithful to Dru for 100 years, even when she gave him plenty of reasons to leave her. He saw her as she "really was' the first time that he came back to their lair and found that Dru and Angel had been having sex. That's just about as good a reason for him to walk out on their relationship as you can get, and he still stays with her. We see at least one more example of the same thing with a chaos demon later, and still, he stays with her. That's not the kind of a man who's likely to leave Buffy, no matter what she does to him. We saw that in Season 6 after "As You Were." Spike was still there, still trying to be with Buffy. The only difference, if that were to happen now, is that, with a soul, "Seeing Red" won't happen again. Call that pathetic if you want to, but the fact remains, Spike is as faithful as an old hound dog. No amount of abuse is apt to run him off.
As for Buffy, when have we ever seen her actually run any lover off? Angel left on his own, at least in part because of his curse, and because he wanted to do more than just be Robin to Buffy's Batman. She certainly didn't want him to leave, She even, literally, begged him to stay. Spike seems perfectly content, even happy, being that very thing.
Riley made his own decision to leave. Maybe he felt that staying and being nothing much more than the Slayer's personal snuggle puppy was just less than he wanted from a relationship, or maybe he felt that he wasn't getting the opportunity to "be all that he could be" there in Sunnydale. The jungles of South America and the mission there gave him more of a feeling of accomplishment. I really can't see Spike leaving under the same circumstances.
I honestly don't think that Buffy could run Spike off with a stake and a flame thrower, any more than I can see Buffy giving up on their relationship after so short a time, if ever. Maybe I'm wrong about that, maybe it's just my Spuffy colored glasses blinding me to the truth that's right there in front of me. It just seems that the two people who have the most doubts about Buffy's relationship with Spike having any chance of lasting are their former lovers. That might be seen as personal experience, but it might just as easily be plain old jealousy. Personally, I think that it's the latter.