Spike is all about artifice, putting on a brave face, trying to be a badass. That's Spike. His created persona, determined to be the opposite of William.
While watching bradcpu's video about Spike's character Bullet with Butterfly Wings (perfect song choice - "Despite all my rage I'm still just a rat in a cage" "Can you fake it for just one more show"), with the word "show" and all the images of Spike in different clothes, something crystallized for me about Spike's character arc in Season 7. I mean, Spike even outright says that he's trying to reclaim himself when he puts on the costume blue shirt in Beneath You, but the new costume doesn't fit. He doesn't know who he is. He can't be Spike--that's the evil persona, right? Spike is a very bad not-man. So the Spike we see from Lessons on is Spike that has been stripped of... well, Spike. His showman's persona of badass. His persona of defiance. Because Spike is defiant of all that he saw William as--snivelling and weak and passive and inconsequential.
But isn't that who Spike is in the first half of Season 7? He's emotionally overwraught and much ado without any real follow through. (In what ways is Spike similar to William? William living in his mother's home. Spike living in Buffy's. His mother pushing him to find a woman. Buffy pushing him to find a calling, a purpose.) Spike at this point has been manipulated and used and downtrodden. Defiant? No. He's accepting, he's been beaten. The most he can manage is devotion to Buffy, repentance. That belief is what gets him through. "She believes in me." It's not that he believes in himself, but that she does.
Then comes Get It Done where Buffy kicks Spike into gear. He goes and reclaims his black leather coat and kills the demon. Because a "tussle like that is good for the soul." But really, it's what's good for Spike. Defiance. Most specifically, defiance of his fears. Of being inconsequential. Of being weak and sniveling and a waste of space. Of being useless. Of being used.
Spike and his persona are all about defiance. Which I think ties into his obsession with Slayers. When you're a vampire, the only thing you fear is the Slayer. When everyone is running from this great and fearsome authority, Spike is seeking her out. To defy her. To win. To prove he's the Big Bad. That he's not weak and sniveling (like William). That he's not to be trifled with. It's the very fact that he recognizes a Slayer's power that draws him in. She's the only one left for him to fear and he sets about defeating her. Unbeknownst to him, Buffy is a different Slayer and she'll be the one to find Spike's Achilles' heel--he's a fool for love.
Reclaiming the coat is about reclaiming defiance. Defiance of fate, of fear. Spike will not go quietly into the night--no, he'll laugh as he burns. And then he'll return as a ghost, and once again fight to defy his limitations in Hellbound. Again, he will not go quietly. Then he'll have his body restored and he'll defy Angel's belief that he's destined for the Shanshu--Spike both defies Angel and fate by drinking the cup. Eternal torment? He's not afraid. Not if that's the price for glory. Defying Angel crosses over into saving Angel and then working at Angel's side when he sees Bigger Bads that need defying. Even defying Fred's demise--"Not this girl. Not this day."--until the futility of the fight is driven home. And doesn't it then make sense why he's the first one to raise his arm when Angel proposes they thumb it to the Senior Partners? If there's one thing Spike gets, it's defiance.
I think this is why one of my absolute favorite scenes ever is of Spike heading to Buffy's at the end of Fool for Love. With a shotgun. Because it starts off with him in ultimate defiant mode and ends with him as a fool for love. It tells you everything you need to know about his character all within that moment when he raises the gun, then sees her tears and softens. For all his defiance and how it ultimately defines him, in the end his heart comes first. It's such a profound moment and the true power of it is conveyed in silence. That is just... neat.