"You know why. I'm not leaving my brother alone out there." Sam is dependent on Dean's touch and closeness after the wall falls - Dean's presence reminds him of why he chose to wake up, and keeps the memories at bay, allowing Sam to function. The brothers have to face up to what happens when their Winchester codependency becomes literal, and the physical, spatial and temporal boundaries of their bond blur the line between familiar and suffocating, comforting and limiting.