Control Issues
Xander doesn't exactly know when it happened. One day, the deadliest vampire in Sunnydale was living in his basement, chipped and soulless and kind of pitiful. And Xander didn't like pitiful - it reminded him too much of high school. And then, the next day, he was down in the basement getting his laundry and Spike grabbed him by the shoulders and pushed him up against the wall.
"Ah ah ah, no biting!" Xander leaned back, his head rapping on the cinder block hard enough to make his ears ring.
"Not biting," Spike said just before he kissed Xander.
"Wha-mumm-ppph-!" And before he could push Spike off, Xander realized something - Spike was a really good kisser. And not only that, he realized he was kissing Spike back. And that Spike really knew how to do that thing with his tongue and -
Spike pulled back. His eyebrows wagged. He smirked and slouched and poked Xander in the shoulder at the same time as he trailed his gaze down Xander's torso. Xander yipped and tried to cover his crotch. Spike batted his hand away. "Hello there, lovely." His smile grew.
And that was only part of it but it was certainly a beginning, and as beginnings went, well...Xander had always attracted demons. And Buffy and Willow - and Giles - thought it was a little different but all right, and Anya sniffed and said she'd seen it coming and Spike looped an arm around Xander's neck and hauled him close to lick his ear and all conscious thought just vanished.
So that's how Spike moved up to Xander's bedroom - at least until his parents found out and they kicked the both of them out. But that's another story for another day.