The answer is so obvious, I'm surprised that no one has guessed it up until now. The Scottish castle that the Slayer crew live in was once the home of a brave young nobleman hho was cursed to wander the halls as a ghost until he was able to accomplish one act of honor for a friend. He and Spike met in the Netherworld while our favorite vampire was in his incorporeal state at Wolfram and Hart, and they became good friends. When Mackensey (that's his name, by the way) saw Buffy's plight, knowing that his friend Spike was her one and only true live, he went back into the Netherworld, retrieved Spike, who was corporeal, of course, but could now travel between our world and the Netherworld, brought him to Buffy's side, and in a reenactment of the Sleeping Beauty legend, Spike kissed Buffy, awakening her from her mystical coma. Realising, of course, that their time of eternal bliss was not at hand yet, and that he was needed back in LA to keep the Brooding One out of trouble, Spike slipped away before the lights could be turned on once more. Because of his unselfish ack of reuniting the two lovers, if only for a moment, Mackensie was granted release from his curse and was given his much deserved eternal rest.
That's my story, and I'm stickin' to it.