I was thinking about this thread and some thoughts jumped out regarding the metaphorical meaning of the recurring phrase “beneath you” and about some of its consequences (maybe I’m stretching them out a bit, but I’ve always had questions about this recurring topic).
1. First, within the FFL episode, we are presented with two pieces of information. Cecily rejects William, effectively rejecting the man. Buffy rejects Spike, effectively rejecting the demon. I think that Spike has to face the rejection of his human self and his demon self. He’s pushed away (by them) because he’s a lesser entity. More importantly, the rejection always comes from the object of his affection: Cecil and Buffy (although his vampire mother and Drusilla also reject him). These rejections can explain his extremely enraged response: getting a weapon for killing Buffy.
2. The show, however, purposely uses the phrase in several occasions after that, the most recognized one is The First’s motto “From Beneath You, It Devours,” which is reinforced by having a doggy-eating worm when first introduced: “there is something coming soon, and it comes from a place of evilness, lurking beneath us.”
3. The other well-known occasion is the episode “Beneath You.” In the church, Buffy tries to touch the chest of shirtless Spike, he recoils as Buffy attempts to touch him. He pushes her hand away and then goes on rambling about only being “flesh” to her. Ensouled!Spike *finally* realizes that he was merely convenient in the past (just as Buffy stated in S6). In other words, he accepts that he has been beneath Buffy all along: “She’d never lower herself that far” (having a real relationship with him -- Seeing Red S6). But more importantly, after getting his soul he also believes that he’s still beneath Buffy, possibly at the same level of the Big Bad lurking in the shadows:
Spike: And - and now everybody's in here, talking. Everything I did...everyone I - and him...and it...the other, the thing beneath - beneath you. It's here too. Everybody. They all just tell me go...go...to hell.
4. Now, moving into the last scene of the episode where Spike reveals that he has a soul, he ends his monologue by asking Buffy if they can rest now. This phrase brings to mind the song of Spike in OMWF: “he wants to rest in peace.” However, the only way for resting in peace (or being "free of life" -- Gone S6) is by dying and thus by being placed “beneath people,” aka buried.
5. I also see another metaphor tied directly to nature of demons in the Buffverse. As a vampire, Spike is beneath humans by definition (“the animal inside? Always the same” -- Potentials S7). By introducing a trigger into Spike’s mind, The First is making use of the Monster/Ensouled!Spike, who is effectively still beneath humanity (still not a man, not a monster -- Smashed S6).
6. Lastly, a slightly more complex metaphor is Buffy’s statement that she feels “beneath them (boyfriends and friends)” because she feels ashamed of feeling superior. But we could also interpret that phrase as Buffy perceiving herself as a lesser being too and it might later help her accept more easily the revelation that she’s given in “Get it Done:” her power is indeed rooted in darkness, which is beneath us.