I believe that Spike has a certain amount of respect and affection for Joyce, born from the fact that Spike was human once and he was a sweet, kinda nerdy, love-poetry writing little victorian boy. His mother was the most important person in his life and her death made Spike terrible and cruel . But that’s what make him so unique , contrary to other vampires who lose their humanity , Spike kept it . His actions are driven by human emotions and it consumes him. He may be dead, but Spike is the quintessential human.
He seeks Joyce , a motherly figure , for comfort after his break up , talks to her and even laughes with her. He feels accepted around her. Her death affected him whereas in theory it shouldnt. To understand Spike , you have to understand William. They are not opposite , just two sides of the same coin.
“Care? Joyce was the only one of the lot of you that I could stand ! She was decent. She didn’t put on airs. She also had a nice cuppa for me .And she never treated me like a freak.”