I Had to Let Him Go.
Dean remembered Sam's first day of kindergarten. It was the first time Sammy would be away from Dean and John. But Dean knew he had to let Sam go.
Dean remembered Sam's first party. He knew what kids did at those parties, but he also hoped Sam was more responsible then that. He nursed Sam through his first hangover, reamed him out the next morning for being stupid. But he knew he had to let Sam go.
Dean remembered Sam's first day of high school after Dean graduated. Sam came home beaten up. He got into a fight with a bully who had left him alone because Dean was always there. Sam begged Dean to let him fight his own battles, and as much as Dean didn't want to, he knew he had to let Sam go.
Dean remembered the first time Sam got hurt on a hunt. A Wendigo in the Minnesota woods cut him up bad and Dean had Sam's blood all over him. He was holding his little brothers hand when a nurse told him they had to take Sam up for surgery, and as much as he didn't want to, he knew he had to let Sam go.
Dean remembered finding Sam's Stanford acceptance letter under Sam's pillow. He begged Dean to understand that he had to do this, and Dean said nothing. When his dad came in and told Sam not to come back, Dean got into the Impala and drove Sam to the bus stop. He told him good luck, gave his little brother a hug and got back in the car. As much as he didn't want to, he knew he had to let Sam go.
Dean remembered when he came and got Sam from Stanford to find their dad he knew Sam had to be back by Monday. He had missed his brother and loved hearing his voice, his laugh, and his attitude again and he thought for a split second Sammy might stay. But after Jericho, Sam said he had to go back, and even though Dean didn't want to, he knew he had to let Sam go.
Dean remembered when he sold his soul for Sam. He wasn't about to let his brother die. He knew Sam thought he was going to find a way out of this deal, and as much as Dean hoped that was true, he knew it was most likely he was going to go to hell. He didn't mind, as long as it was for Sammy. But he knew he better do what he could for his brother this year, because when his year was up, Sam was going to have to let him go.
Dean remembered when Sam broke the last seal. The apocalypse was in full swing and Sam knew what he would have to do to stop it. Dean yelled when he told him, but then once he calmed down, he talked to Sam. He told Dean that he let the devil out, and he had to put him back in. It went against ever fiber in his body, but Dean knew he had to let Sam go.
Dean remembered being angry at Sam for not looking for him when he was in purgatory. He found a girl and settled down instead. After Castiel killed Samandriel and they finished talking that out. Dean told Sam to figure out what he wanted. Both feet in or both feet out. Dean or Amelia. And Dean knew that if he chose Amelia, he had to let Sam go.
Dean remembered when Sam had to go into hell to rescue Bobby's soul. Dean didn't want him to go alone. Sam argued, saying he could handle it, that he could handle purgatory as well. And as much as Dean hated the idea, he knew he had to let Sam go.
Dean remembered Sam lying in that hospital bed after they decided not to finish the trials. Ezekiel showed him Sam talking to death, saying that he was ready to go if it meant nobody else would get hurt. Dean convinced Sam to let an angel possess him, because as much as Dean hated the idea of Sam not knowing he was possessed, this time he damn well wasn't going to let Sam go.