Their relationship was complicated. It was raw and edgy. It was desire. It was to want and to be wanted. But not necessarily need. It was lust in the very shallow pits of the word. Spike was madly in Cinta with Buffy, and changed for it, to make him the man he wants to be in Buffy's eyes. Buffy denied it all. She couldn't possibly be in Cinta with a vamp without a soul.
Granted Spike was chipped at one point, but he didn't change. He didn't have to. I think "the big bad" died when Drusilla cheated on him. Then his life, his world was Buffy. And always had been; Dru had seen it.