I was thinking that robo!Sam didn't particularly care about getting his soul back - or perhaps still doesn't - until Crowley said he'd send Sam back to hell. Maybe it's more about not going back there, which I expect Sam can remember as a Very Bad Place than it is about becoming okay again. Or perhaps it simply gives him direction and purpose, like hunting for the alphas does. Though the alpha-vamp did say that Sam could feel his emptiness, which probably doesn't feel good... perhaps he'd like to fix that?