I do agree they haven't done a good job of explaining Sam's motivations. Part of that is the "mystery" about him, now more than ever, but still... there needs to be more onscreen and less fanwanking.

I've been pondering. Perhaps Bobby seeing this and how Sam reacted, as well as having some insight to how he was acting while Dean was in Hell might come into play? He's not with them all the time, but Bobby is pretty damn paranoid (not without good reason) and might figure out that things are not quite right. I wouldn't be surprised if he's already got an action plan in mind in case things to to (pardon the pun) Hell.