Then again with Spike when Joyce died he did miss her and the flowers had no card so they weren't an attempt to have sex with Buffy, as for getting his soul only to get Buffy back, I think that was part of it but not all of it, perhaps he did it partially to prove that Buffy was wrong about him and all of that...and if Spike is a bastard then what does that make Angel...he's no choir boy after all...
Not to mention the bits with Glory where she tortured him but Spike never gave up Buffy's secrets and then later on when he thought that he was with the robot he again protected her, point being that for both of those times Buffy would more than likely never have known what he'd done for her...and Spike tried to help Dawn out when she found out the truth, yeah he didn't do it well but still...