Spike & Buffy
Spike and Buffy....(laughs) Too amusing! But bizarrely true. Sure, another vampire and the Slayer. Been there, done that. But this is *Spike*.  Spike who spent the majority of Season 2 trying to kill Buffy.
It's completely understandable why he wants her. She is the only one he can't defeat, and never have. She is stronger than he is. And he knows it. Besides we know Spike has a thing for Slayers, his bloody past showed us that much.
But now it isn't a matter of killing her, Spike wants to consume her. He wants all of her, to have and to control. Essentially she contols whether he lives or dies. She could kill him tomorrow, and that excites and frustrates Spike. Poor boy, his head's all confuzzled.
He knows that wanting Buffy is wrong, but he is drawn to her power, to the control she has over him. The control that he is fighting for.
At the end when he cries for her, is so endearing and touching. He cries for what he never could have and what he never will have. She put him in touch with his human side. He wants to impress Buffy, shows that he can be anything for her; just like Angel was. Spike has always been jealous of Angel, first over Drusilla and now over Buffy. By having Buffy Spike is getting one over on Angel.
Being in love with the  Slayer was the only thing that Spike could do. After all, he was around all the time, he saw the way she fights, and how hard she fights. By having this chip, Spike was automatically put against his fellow demons. When he realised that demons were fightable that made him useful for Buffy. Though not her ideal company, Spike's vampire strength replaced that which Angel's used to fill. I suppose Spike could have seen himself as replacing Angel, in everyway that Angel was a part of Buffy's life, Spike wanted to be a part of her life too.
Eventually, when Spike kidnaps her, his motives were all about love; or in Spike's case obsession. Buffy doesn't need nor want Spike in the way that he wants her. She was rather mean about it though. It can never be between these two.
Spike has issues with women, he needs to consume them; to be in control. He never had that with Drusilla. And he sure as hell doesn't have that with Buffy. His obsession leads to an insane obsessed mind, doing insane obsessed things....like the BuffyBot?!
Towards the end Spike gains Buffy's respect. His willingness to help Buffy with Dawn makes her able to trust him. She knows that he would do anything to protect that which Buffy holds dear. Mainly Dawn.
Buffy made Spike human. As human as a vampire can be. Love will never be a part of their relationship, and Spike will never stand a chance with our Slayer. But I can see trust and respect....perhaps friendship? Who knows? We shall see.

Season Six resurrects Buffy from the dead. She and Spike now have the ultimate bond: they have both died. She knows he understands this in a way none of the Scoobies ever could. This brings them together. He still loves her but he wants her on his terms, all or nothing. She needs to love him for him, not for something else.
Will Spike ever know the love of the Slayer?
Buffy's resurrection has left her numb, Spike has been through death so they bond. Eventually the only way she can feel anything is to have sex with Spike. Buffy uses him to make herself feel something. She doesn't love him, she can't. Spike tells her that she came back wrong. That she belongs to the darkness, and should be with him. They have a violent and interesting sexual relationship, but no real friendship or love is involved. When Buffy realises that there was nothing wrong with her after all, she breaks up with Spike. He keeps her in the darkness and she needs the light to live, and that's the one place Spike, like Angel, can't take her.