It was only a moment, just that one second of clarity. Sam was leaving him and there was nothing he could do about it. He watched from the doorway as Sam packed his bag but the image he saw before him was much different. Sam was shorter, smaller, younger when he turned around and gave a small smile. Blinking away tears he saw the man before him, the brother he helped raise and the one that he pushed away. His throat swelled threatening a sob as Sam picked up his packed bag and turned to face him. Neither of them were smiling, they were both afraid of what would happen. Dean had never been without Sam like this, there was no coming back for him. He knew that.
A car honked outside of their cheap hotel room, the precise moment their Father walked into the room. His shoulders were tense and he gave one look at his youngest son and turned back around and left. Dean wanted to call out, force him to say goodbye to boy, and looking back he saw that Sam wanted to run after him. Taking the steps that separated them, Dean wrapped his younger brother into a fierce hug that was returned. The honking outside persisted and Dean was forced to let him go. Pulling away, Sam placed his hand on his brother's shoulder. There were tears in his eyes and with a small smile he was gone.
Hearing the car drive away he listened as his baby brother went off to live a life without him. Now he was left alone and scared, because he made the mistake of letting himself need someone else. He let himself love someone he shouldn't have.