Disclaimer: Joss is the boss, I'm just a fan.
Warning: Contains a lot of sexual innuendo. Don't like it, don't tell me.
All's Unfair in Love and Lust
Spike was 125 years old. Most vampires didn't bother counting, but Spike wasn't like most vampires. Sure he drank blood, wore leather, and gelled his hair, but there was one thing that separated him from other run-of-the-mill vampires. Every other run-of-the-mill vampire wasn't in love with the Slayer. In fact, there was only one other vampire cursed with the same affliction as Spike but he lived in L.A. and Spike didn't like to think about him.
Not to say that Spike had always loved the Slayer, far from it. In fact, the reason he had even come to the hell spot (literally) known as Sunnydale was the precise business of killing one Buffy Summers, A.K.A. the Slayer. It was only after countless failed assassination attempts and a cookoo science experiment that he realized his raging paranoia had caused an almost schizophrenic reaction, which in turn had led to the knowledge that he was in love with the object of his blood lusting desire.
Under ordinary circumstances, vampires didn't fall in love, but as was previously established, Spike was no ordinary vampire.
Not to say ordinary vampires didn't feel lust, in fact quite the opposite. There is nothing more--for lack of a better word--erotic for a vampire than sinking their fangs into another creature's flesh. The feel of their warm blood sinking through their cold body could only be described as orgasmic. Some vampires were choosy whom they bit for this exact reason. Like a person who prefers blonds to redheads, some vampires preferred a certain blood type to another. Spike had always been partial to O-. Despite its rarity and perhaps because of it, he always had to repress the urge to turn on his vampire face whenever he caught wind of it as far as a mile away.
It can come of no surprise, therefore, to learn that Buffy Summers was O-.
Typically this would be ideal. Like vampires in similar lust situations, Spike would simply have to bite Buffy and they'd both live happily ever after.
Unfortunately there was a problem: Spike didn't just love Buffy, he loved the Slayer and one just doesn't kill the Slayer even for such a noble cause as love. One kills the Slayer for revenge, for respect, for fun, but never for love. It never ends well. Slayers made notoriously bad vampires. The last time a vampire had made the mistake of turning a Slayer, she had gone on a rampage and killed not only her sire, but also all of her sire's friends before finally going crazy and killing vampires and humans alike on some kind of holy quest. She was eventually caught by a group of angry French villagers and burned at the stake. That had been around the year 1431 if Spike was not mistaken. Joan of Arc, she was called.
As much as Spike desired Buffy, he couldn't do that to her. He couldn't turn her into the one thing she despised above all else. He couldn't make her like him. Partly because being a vampire was not all it was cracked up to be, but mostly because he knew she would be pissed beyond all Hell's fury if he tried.
No, there was no doubt about it; there was no cure for his sorry condition.
And so Spike resorted to annoying the Slayer to the point that she would eventually replace the metaphorical stake in his heart with a real one, thereby putting him out of his misery, while in the meantime dreaming of her tripping and accidently landing on his fangs.