Buffy is coming (reluctantly and secretly) to the conclusion she's accepted a lot of things about vampires that are, to put it mildly, bull.
Or at least that vampires are more complex and varied than anyone is willing to admit. That some of them (or one of them, it's not like she's done an exhaustive study here) can be startlingly human under the right circumstances. It goes against everything she's ever been taught.
She's always been more inclined to trust what she's seen than what she's been told, but what she's seen recently has turned a lot of received wisdom so far on its head, it's kind of hard to wrap her brain around it, despite the evidence of her own eyes.
Spike comforting her sister. Spike grieving with them, sharing memories of Joyce, of "She was a proper classy woman, you know, always treated me right, always ready to listen. Really special bird, your mum was. Nice lady." A fond smile. "Made a cracking cuppa."
(He's also been sweet with Tara and sympathetic toward Willow, which is way more difficult to write off as 'trying to get in Buffy's underwear'.)
He's actually made Dawn smile, even laugh. Given Dawn's being pursued by a hellgod who wants to use her blood for inter-dimensional travel and a bunch of mediaeval knights who want to kill her, not to mention they're both still reeling from Mom's death, Buffy's grudgingly impressed. If it didn't sound completely ridiculous to say such a thing about a vampire, she'd be inclined to think he had a soft spot for Summers women in general, not just some weird crush on her. And maybe it's true even though it's ridiculous.
She's not sure what happened exactly, but she knows he helped Dawn, helped and comforted her (and was probably a horrible influence on her) back when Buffy was clinging alternately to all the distractions of burying her mother and to Angel.
(The fact Spike hasn't boasted about it suggests it was either something nefarious or something done genuinely for Dawn's sake. Possibly both.)
The idea he's in love with her, well... it's nonsense, Buffy's still holding on to that. But she can't deny he's got some kind of... of genuine affection for her? And for Dawn? God, she can't begin to make sense of it, and it's really strange, but it's something, something she can't ignore any longer, however much she'd like to, however much it goes against everything she was so sure about.
Trying to outrun a severely demented and scary powerful Glory is taking up most of her attention, because she does have a little common sense, but her mind is point blank refusing to give the Spike thing a rest. There are too many things she can't explain except by admitting that what she thought she knew was wrong, and it's not that she's especially bothered by 'being wrong' (well okay, maybe just a bit), but she doesn't know what it means or how to handle it if she is.
Spike is nothing but a vampire, without real emotions, supposedly. So how is it that he instantly understood what Willow was thinking and feeling when Tara got brain-sucked? Understood so much better and more clearly than Buffy, who's known Willow for years?
Willow's her best friend. The First Slayer said Buffy loves with all her soul. Buffy doesn't understand how her soul can mean very much if she missed something so vital, and Spike - Spike of all people - knew, as if by instinct.
'I'd do it. Right person. Person I loved. I'd do it.'
The memory makes her wince. Spike's not great with the whole subtlety thing; his heart is on his sleeve and all over his face even when he's a walking bruise. (She still can't quite get over the beating he took.) The subtext wasn't very subtexty. She knew exactly what he was saying, and it's getting harder to discount. (Though she hasn't stopped trying, not yet.)
She's found herself relying on him without meaning to, trusting him to help her even as she dismisses him as 'just a vampire'. Taking Dawn to him was... automatic. Just like she'd taken her mom and Dawn to him before.
And then he went and got all insightful about Willow, and now he's helping her again, helping her keep Dawn safe, helping her run away. The chip stops him biting them, but it wouldn't stop him hurting them indirectly if he really wanted to, and it definitely doesn't explain him helping them, putting himself in danger for them.
So the next logical step is to conclude she was wrong, that vampires may be a lot more complicated than she thought. And that is... kind of a pain. It's not likely to help with the whole slaying gig if she starts worrying about the monsters she's supposed to be turning into piles of dust.
And worse, or at least a lot more confusing, is Spike himself. And maybe he's the oddity (part of her hopes so, because she'd like to keep vampires in general in the column marked 'always evil, slay on sight'), maybe it's just that Spike is kind of weird (she can totally believe that), but either way... it's really not the point. The point is that maybe he has real feelings, and certainly he believes he does, and she's been stomping all over them.
Which... well, it's Spike, and he's a royal pain in her backside, and his behaviour was twelve kinds of creepy, so she can't feel too bad about it, but she does feel a little bad. She doesn't like being actively cruel, and although he's not her favourite person, her hatred for him has long since turned into more of a tolerant disdain, and... she could so do with not thinking about this right now.
So she tells herself she'll put it off, that she'll think about it when they've dealt with Glory. Maybe when she's got a tiny bit of emotional energy to spare, she'll be able to figure out what it means. And maybe figure out what the hell she's going to do about it.
In the meantime, she's trying not to let anyone know what's going on in her mind. She's pretty sure they'd all want to have Words with her (it's not like they were exactly wild about accepting Spike as part of the escape plan in the first place), and she's just so tired right now, and if one of them tries it, she's liable to commit her very first intentional homicide.