Someone was talking about Sam walking a thin line living with Dean, hypervigilant and mortally afraid, but still sticking with Dean in the hope of mitigating the damage that Dean’s capable of—well, here it is.
My own gut twists with unease and fear when I think of that final scene of the last episode (where Dean refuses to divest the blade, no explanations, no excuses) in conjunction with this scene. Sam is terrified.