THAT'S NOT SAM'S SOUL! IT'S ADAM'S LUCIFER ADAM'S! He won't tell Dean the truth because he doesn't want to go back to Hell.
Also, Sam totally has soul!herpes now ("try not to scratch it"). He got it from Lucifer. During one of the shower scenes in Hell. Yep.
Unrelated, but I swore Bobby was gonna die of a heart attack after Dean stopped Sam from killing him. Oh, irony.
Wow. That was a lame theory, but I'm still trying to finish up my last final paper, so don't blame me. Blame my overtaxed brain.