Sam walks out again.
Dean tears himself up, he drinks until he’s numb and limp on the floor, and he sends Sam voicemails ranging from thick-throated apologies to demands that he stop being a coward.
Sam comes back, he tells Dean he won’t hesitate to leave if he fucks up again, and he agrees, just glad to not be alone.
Dean will never know what wire he’s tripped, but he knows that he was wrong, that he’s always wrong and Sam’s right to leave.
The cycle will repeat until one of them dies, and he finds comfort in that certainty.