Multiverse theory states that there are infinite universes out there. One that Sam Winchester maybe had too much smoke inhalation for his tiny six-month-old body to take. That doesn't stop him from keeping his brother company...
For a four year old that just lost his mother and baby brother in a blaze of unnatural fire Dean was doing the best he could. Lately, his Dad, who now wanted to be called 'Sir' or 'yes sir' was too wrapped in things that he didn't understand. Dean understood that he didn't have a home anymore in the traditional sense the Impala being the closest thing to something that didn't get left behind or get destroyed. Dean had friends and people that he knew and loved, but they were dead or left behind in Kanas. All but Sam.
Sam was gone, Dean knew that, but that didn't mean that Sam stayed away. The six-month-old never left him alone he watched Dean and babbled baby talk as they rode around the country. Dad never said anything and when Dead talked back to Sammy John always told him to shut up. Dean took to talking to Sammy when he was alone which was a lot actually. He understood that his Dad had an important job and that Sammy and Mom needed to be avenged. Those facts didn't stop him from missing his dad. When Dean made his own dinner he asked Sammy what he should make just to make conversation the baby laughed and Dean made what he always did cold cereal or something else that didn't need a stove or adult supervision.
Sammy was the only company he got for weeks at a time. It was slow at first, but Dean noticed that he was growing with him. The ghost Sammy was getting taller and talking in real sentences. No one, but Dean ever saw him though. Sammy was nice though, he was there when John got drunk when John taught Dean how to give stitches when Dean was taught to handle every weapon in the arsenal against the supernatural. Cheering him on and making sure that he was safe. Sammy always watched his back and saved him more times than Dean could ever count.
As Dean grew older and John let him go on solo hunts he never was truly alone. Sammy was there to tell him that the werewolf was 'just up ahead and that he was super angry' or when Dean got hurt Sammy stayed up with him when sleep was eluding him. Sam and Dean were inseparable even when one of them was long dead.
When John and Dean fought and thought it best to take more solo hunts. Dean longed to have someone in his corner. Sammy was there, but it wasn't the same. He couldn't put a reassuring hand on a shoulder or give a hug on a rough hunt. Dean didn't mention it and Sammy couldn't remember a time when he had a real body that felt and tasted things. He could remember his older brother. He remembered being small enough for Dean to pick up and the panicked look on his Dad's face when they saw he wasn't breathing anymore. He never regretted that he hadn't been at peace enough to leave the earth. He liked keeping Dean company. It was selfish, but he was never lonely with Dean around, even as a twenty-something ghost that had a tie to his older brother.
They never figured out why John couldn't see him. Dean as he got older, didn't mention it for fear of an unwanted salt and burn. He would do anything to protect his little brother he lied on a daily basis, so really what was one more to his Dad?
It wasn't that bad. Dean was never alone and Sammy wasn't either. People that saw Dean talk to himself thought him odd they never knew the truth. Sammy was always there to answer or give some wisecrack about the situation he was stuck in. Like the time there was a bug curse Sam laughed at him and his makeshift flamethrower. Dean knew if he was flesh and bones Sam would have freaked out, at least as a ghost the bugs couldn't hurt him. When he fought ghost and ghouls it was always in the back of his mind that if he died Sammy wouldn't have anyone. He had to be careful about everything he did because he wouldn't leave his brother alone. He couldn't Sammy needed him just as much and he needed Sammy.