Guys think about it.
Spike died knowing that he was not the man Buffy deserved. But rather the one she needed. At that single point in time, Angel was not there, Riley was not there, nor was the future or past. As his legs were burning away, then his arms, and finally his smile, Spike was the most important man in the universe. The hero. He was finally Buffy’s dark knight. So in his last moments, before he turned to dust and took the whole town with him, Spike smiled.