Buffy, as a slayer of vampires, provided a significant reversal of fortune for Spike. His rivalry with Angelus also didn't always prove profitable. This is probably why both of them became objects of such obsession for Spike early on.
I mean, Buffy kinda specialized in making ancient or powerful vampires look like morons. Look at Darla, for example.
And this is pretty much the single worst town in which to be a bad-ass creature of the night. If you're not getting your ass kicked by Buffy or Faith or Angel, then the goddamn initiative is coming after you. Keeping your head down becomes the move for most of the independent denizens of evil.
But Spike was pretty much a loser in his mortal life, so this is kinda par for the course.
We haven't gotten any real back story on spike just yet, he just roles in takes over and kills the annointed one, then basically fails in every endeavor he has up to the point where I'm at. I just finished the first part of the two parter with the black slayer, I assume he'll actually win this time and kill her.
And don't forget Buffy only beat Darla with the help of Angel, and even then I didn't really think she stayed alive for so long because of her own cunning but because of her connection to the master.
It's strange though, the series starts with Buffy moving to the hellmouth which we are told is the place of the worst magical juju around. Well what happened prior to Buffy being there? If there is only one slayer in the world what happens with all the other vampires and demons around the world?
I'm sure some of this is explained as the series progresses the the mythos is expanded upon.