When Spike appeared in Sunnydale for the first time, it was his only plan to kill Buffy. And if Joyce hadn't been there at the right moment, it would have worked at the first try.
And after that every plan failed terribly. When Angel lost his soul, stole him Drusilla and then tackled with her the destruction of the world, Spike asks Buffy for help. Buffy mistrusts him, but then she agrees.
At his second return to Sunnydale, all he wants is the ring of Amara to get rid of Buffy once and for all. 'Cause after all she's the one who ruined his whole unlife. At least that's what he thinks. Long time before Drusilla already knew that Buffy impressed Spike in a totally different way.
Of course, he fails again. And even against Angel and his friends he has no chance. His anger is growing more and more, until he gets caught by the Initiative. With the chip in his brain, he's not able to show himself at his fellow vamps, even Harmony is pissed off. Buffy is his last chance. Although she doesn't think about helping him at the beginning, at the end she does it anyway - maybe out of pity, but mabye only because she wants some info about the Initiative.
Their little love affair caused by Willows spell didn't improve their relationship that much. But at least he was allowed to walk around freely and help the Scoobys with their work, even when he always had the opposite in mind.
After a dream he had one night Spike realizes that he'd rather kiss the Slayer than kill her. Carefully he tries to approach her, he helps and comforts her, but with all her worrying Buffy doesn't notice. When he plucks up all his courage and tells Buffy about his feelings, she blocks and doesn't want to see him again - ever.
But soon she realizes that Spike is the only one who can help her fight Glory and protect Dawn.
Even after Buffys death he keeps his promise and cares for Dawn.
When he sees her again it is 'the happiest moment of his entire existence'. Buffy trusts him and tells him everything, 'cause he understands what she's been through. They spend time together, talk, drink.
Another spell in Sunnydale forces all people to admit their secrets and feelings, and at last the one thing happens Spike is waiting for: they kiss. And they do it several times. Buffy is disgusted, but she can't resist him. The current climax of their relationship is a night they spend together - some hot sex in a collapsed house ...
After that night they spend a lot more comfy time together, but Buffy doesn't want anyone to know. All the time Spike tries to make Buffy say that she loves him, but of course, she never does. When Riley visits Sunnydale with his new wife, Buffy realizes that she's only using Spike and breaks up with him.
Spike is angry and sad and at a moment in Buffy's bathroom when she's telling him again and again that she wants him to leave he freaks out and tries to rape her...
When he realizes what he was about to do, he's shocked and leaves Sunnydale. (to learn about what happened in the meantime click here)
When he comes back to Sunnydale Buffy doesn't realize why he has changes until he tells her that he got his soul back. It takes her some time, but she begins to trust him again. When he tells her he wants to leave again, she says that he has to stay, that she's not ready for him to leave yet. She is the one to lets the Initiative people remove his chip and she tells Director Wood and Giles that they should never lay a hand on him, 'cause she needs him to stay and to fight.
Their relationship grows stronger, and there's not only sex, but also something like friendship. Spike is the one who gives her the strength to get up again and to keep on fighting.
Buffy gives him the medallion she got from Angel which may only be worn by a true champion. When during their last fight the meaning of medallion becomes as clear as the fact that Spike won't survive, Buffy finally says the words that he always wanted to hear. But he knows that she doesn't mean them...

"I love you, Spike!" - "No, you're not. But thanks for saying it!"