Boo Much Fun: UST means "
unresolved sexual tension". It's a very common trope in fiction. There's also a quite common subtrope of
belligerent sexual tension.
And if you can't connect this to real life... didn't you ever see boys in school pulling the pigtails on girls they actually have a crush on?

Honestly, some of Buffy/Spike interactions, in season 4 in particular, came off looking as a more extreme version of two kids picking on each other in the playground.
(Cut to Giles' bathroom. Buffy is sitting on the end of the bathtub in which Spike is chained up)
Buffy: (Exasperated) So..you saw their faces but you can't describe them.
Spike: (Playing coy) Well, they were human. Two eyes each, kind of in the middle.
Buffy: Uh huh. And the lab?
Spike: Underground. I came out through an air vent. I don't know exactly where. I'm done. Put the telly on.
(Giles enters, carrying a mug that reads "Kiss the Librarian" with a straw protruding from it. It contains blood)
Spike: It's about time. Hope you got it warm enough.
(Giles hands it to Buffy without saying a word. She takes it, sighs, and makes a face as she puts it close enough to Spike that he can suck through the straw. He makes a big to do out of it, so as to disgust her more.)
Spike: I don't know why you're so dainty all of a sudden. You've done this for Angel — you must have.
(Buffy pulls the mug away, leaving Spike with the straw dangling from between his lips)
Spike: Hey! Give it!
Buffy: Okay, that's it. The invalid amnesiac routine is over. The kitchen is closed until you can tell me something useful about the commandos.
Spike: I'm tryin' to remember. It was very traumatic.
Buffy: How long are you going to pull this crap?
Spike: How long am I going to live once I tell you?
Giles: Look, look, Spike — we have no intention of killing a harmless.. uh, creature.. but we have to know what's been done to you. We can't let you go until we're sure that you're .. impotent —
Spike: Hey!
Giles: Sorry, poor choice of words. Until we're sure you're, you're..
Buffy: Flaccid?
Spike: You are one step away, missy.
Buffy: (Sarcastically) Giles, help! He's going to scold me.
(Spike growls, trying to grab Buffy, but the chains hold him and only make his struggles comical)
Buffy: You know what? I don't think you want us to let you go. Maybe we made it a little too comfy in here for ya.
Spike: Comfy? I'm chained in a bathtub drinkin' pig's blood from a novelty mug. Doesn't rank huge in the Zagut's Guide.
Buffy: You want something nicer? (She leans her head to the side, exposing her throat to him) A look at my.. poor neck? All bare and tender and exposed.. all that blood just .. pumping away..
(Spike, by this time, is all but licking his lips)
Giles: Oh, please.
Spike: Giles, make her stop.
(Giles walks out of the bathroom and into the living room, speaking to Willow who's reading through some books)
Giles: If those two don't kill each other, I might lend a hand.
(4.9 "Something Blue")
Buffy: Okay, drop the act..
(She grabs him by the arm, but he pushes her away)
Spike: Get off!
Buffy: Okay, that's it— I'm gonna gag you.
(He punches her in the nose, then yells in pain. She punches him back in the nose; he yells in pain again. Cut to Giles in his bathroom. He's putting drops in his eyes. Buffy and Spike come barging in through the front door, Spike once again tied up)
Spike: Hey! Watch it!
Buffy: One more word out of you, and I swear..
Spike: Swear, what? You're not gonna do anything to me. You don't got the stones.
Buffy: Oh, I got the stones. I got a whole bunch of .. stones.
Spike: Yeah? You're all talk.
Buffy: GILES! I accidentally killed Spike. That's okay, right?
(4.9 "Something Blue")
AMANDA
Do people ever think you're weird?
BUFFY
Um, (flutters eyelashes, shrugs) I guess. Sure, in a charming, endearing, loveable... Yeah.
AMANDA
I feel like people think I'm weird, and so they pick on me. But I might be—weird.
BUFFY
(chuckles) Amanda, why do you think that you're weird?
AMANDA
One of the boys who picks on me, I kind of—See, if a guy picks on you, is it weird to think he's cute?
BUFFY
(grins uncomfortably, leans back in her chair) Oh.. (sighs)
AMANDA
It's just, the last few times I've seen him, I've wanted to, you know, pick on him....extra. More.
BUFFY
(stifles a laugh) Uh-huh.
AMANDA
The thing is, I can't tell. My mom says when a guy teases you, it means he likes you.
BUFFY
(nods) Sometimes that's true.
AMANDA
Is it weird? We're mean to each other, and we like each other.
BUFFY
Well, it depends. Sometimes that's how people relate. Being mean to each other. Even mortal enemies— (increasingly emphatic) Then with the— And that leads to no good, absolutely no good. And much confusion. A-and then it's over. Absolutely, seriously, definitely over. And that's confusing too. The over part. Which it is. Over! (catches herself, calms a bit) So, maybe.
(7.12 "Potential")
* As DevilEyes pointed out, there is absolutely no on-screen evidence to support the idea that Buffy ever learned the full truth about Spike's human life and the circumstances that led to the careful construction of his current personality and persona. I wouldn't disagree with the idea that Buffy does feel Spike is beneath her, particularly prior to the events of Season 6, but the idea that this was based on some intimate knowledge of his human past as William was never ever stated or even implied on-screen.
Yeah, I admitted that I just assumed Buffy had heard the story, because I don't remember the later seasons very well and thought that having heard the story was the only logical way Buffy could know the exact words that could hurt him most. And I still think the fact that the writers put those words in her mouth without her knowing the story is way too coincidental to seem natural and really stretches plausibility.
Um, it is a show about vampires, demons and witches, you know.
The writers enjoyed playing with the "beneath you" phrase, especially in season 7, when it gained a whole new meaning, as the designation for First Evil: "From beneath you, it devours" which gave a double meaning to the title second episode of season 7 "Beneath You", which was both about Buffy/Spike and about the First Evil, Buffy actually hears Spike use the phrase "from beneath you, it devours". At the beginning of S7, Buffy finds Spike, deranged and manipulated by the First, in the basement of the Sunnydale high school, literally in the underground, "beneath". Ironically, in their last scene together in the series, in the finale, she is literally
beneath him while he's starting to burn, destroying the Hellmouth.
It's not Spike's attraction to Buffy that I find absurd and have trouble understanding, it's her willingness to indulge him at all. There was a lot of time spent with her rejecting and mocking his affections and all of that made sense to me. There were times when it was entertaining and (like the "beneath me" line) also times when it made me like Buffy less because I felt she was being way too cruel, but at least it always made sense.
Let's go back to something I said in response to you a couple of pages back. You really didn't get the impression that Buffy was written and acted as "protesting too much"?
For instance: didn't it ever seem strange to you that Buffy only decided to revoke Spike's invitation to the Summers house in "Crush" after she learned he was in love with her... even though she had never done it in the preceding 2 and a half years (since the season 2 finale, when they teemed up to save the world)? Remember, during that time, he was without a chip, able to kill and hurt anyone he wanted; she found him in her house, drinking hot chocolate with her mother (season 3 "Lover's Walk); he tried to kill her again (season 4, "The Harsh Light of Day") and he was without a chip that time. Then he tried to kill her again (season 5, "Out of My Mind"), but couldn't because of the chip. Then he told her she had a death wish and talked about killing the two previous Slayers, and that he would kill her one day when she slips ("Fool for Love"). In all that time, she never considered him enough of a threat to revoke the invitation? But then he tells her he is in love with her, and suddenly she is completely freaked out - and immediately asks Willow to un-invite Spike? How does that make any sense? Xander laughed when he learned about Spike's feelings for Buffy, because he thought Spike had no chance in hell with a girl like Buffy. But Buffy herself was horrified. Why? What could a chipped Spike in love do to her, that a chipped Spike who hated her couldn't (let alone a non-chipped Spike who hated her)? Why would his love be scarier than his earlier attempts to kill her?
The only possible reason I can think of is if she did feel some twisted attraction for him and was afraid that she could reciprocate in some way.
When she cries to Tara and her friends all have disgusted reactions to finding out about them, I felt like saying, "yeah, tell that to the writers!".
Actually, Buffy herself expected her friends to be far more disgusted than they turned out to be. Tara was very understanding and non-judgmental, Willow was OK with it, so was Dawn (who always liked Spike anyway), and the only one who had the predicted disgusted reaction was Xander - which was hardly surprising, but also very hypocritical, since he had dated and almost married an ex-demon who had been killing and torturing people for a thousand years, and hadn't expressed any remorse for it at all.
Tara (a gay woman) even used the words "You're not ready to come out yet?" in describing Buffy's dilemma whether to tell her friends about her secret relationship.
By that point, however, Buffy and Spike being a couple would just be a retread of Buffy and Angel, so I still didn't like it.
It wasn't a retread of Buffy/Angel, as much as a the upside down version of it, with as many contrasts as parallels. Buffy starts off idealizing Angel, blind to his dark side; they have a tender romantic teenage love; they have sex, he loses his soul, she gets to see the monster that was inside that man all along. (But keeps denying it; no, the man and the monster are two completely different beings - "my boyfriend" and "demon wearing his face" - right? So it doesn't reflect on her... because she was only attracted to the good man - right?... not to his dark past or the danger , as Faith suggested in season 3, which really angered Buffy...) Buffy starts off hating Spike, aware only of his monstrous side; she only gradually starts to see the man that was inside this monster all along, but seems to oscillate between seeing him as one or the other - and even seems unsure to which of the two she is more attracted; they have a dark, destructive adult sexual relationship that ends badly; unlike Angel, who lost his soul as a result of his relationship with Buffy, Spike gets his soul back. There are, of course, still plenty of similarities - like the way she protected and saved Angel in season 2 and took care of him in season 3, and the way she did the same for Spike in season 7, and the fact that she ends up losing them both. But it never going to be the same story, because the naive, bubbly teenage girl Buffy, eager for romance, wearing her heart on her sleeve, was a very different person from the adult, hardened, emotionally closed woman we see in season 6 and 7, burdened with responsability and scarred from too much pain, who found it very hard to connect with people and express any emotion.