Dean was exhausted. Earlier that night he had gotten a call from Sam, saying that Lucifer had approached him and declared him as his vessel. Despite what his heart was saying, Dean decided that he and Sam needed to stay separated. He loved his brother, but if Sam was getting approached by the devil, he needed to keep him as far away as possible. He still had to take care of you–his daughter–he needed you safe and you wouldn't be with Lucifer around. He had been taking you with him from motel to motel, trying to keep you away from armageddon.