Dean was so traumatized by watching his mother burn to death in a house fire as a child that he created an entire alternate reality inside his own head, permanently subdued in a psychologically self-induced coma.
John gave up on Dean ever waking up, so he stopped visiting. Dean imagined John ‘went missing’.
John came back only once to “say goodbye” to his son and finally make peace with letting him go - so in Dean mind, he died.
Sam’s the only one that visits him regularly, because Dean’s really the only family he has left, and after all these years, he still has hope.
When he visits, he absent-mindedly tells Dean the murder cases he’s working on, so Dean imagines they’re hunting.
When Sam graduated law school and broke up with Jess, Dean imagined Sam dropping out and Jess dying. Every time Sam was dealing with real life problems and couldn’t visit for a while, Dean imagined himself stuck in hell or in purgatory, or Sam being trapped in the cage with Lucifer or going off with Ruby. When Sam was really stressed out and ranted to himself in Dean’s room, Dean got worried - and in his head, it translated into addictions on demon blood or going through the trials.
Sam’s “light at the end of the tunnel” and “dreams of a normal life” are based on Sam telling Dean that he thinks Dean can still hear him in spite of the coma, and believing that Dean will someday wake up.
Dean’s inability to see a normal life for himself represent the mental walls that prevent him from waking up.
Bobby was Dean’s specialist for years, and used to sit at his bedside and tell him stories about the mythology Dean’s file said were his favorite, before the fire, in hoping that it might trigger something within Dean to wake up. Dean felt as if they were having long conversations in that room, even though they were one-sided – and imagined Bobby as a fellow-hunter, a go-to guy with knowledge on all things that go bump in the night.
Bobby ended up bound to a wheelchair after a car accident and retired. During his last week of work, tying up cases, he tried his damnedest - one last time - to reach through to Dean. Dean imagined Bobby was a ghost that couldn’t let go. Every time Bobby came back in Dean’s head, his old doctor had come back for a visit to the one mystery patient he couldn’t solve.
Castiel was originally a med student assigned to the team working on Dean’s case, but was eventually certified and took over after Bobby retired. Every time Cas went on vacation, or temporarily covered for doctors working on more demanding units, Dean imagined him dealing with the war in heaven or not answering his prayers. In Dean’s mind, the whip of his white coat over scrubs sounded like the flap of a trench coat.
Kevin Tran was working on his doctorates thesis and chose Dean to be one of his top patients in a study, but decided to move to another hospital after his student research was finished.
Lisa was a patient in the room across the hall, and sometimes her son Ben would wander into Dean’s room and talk to him about his mom when he would visit. When she recovered and left, Dean no longer imagined a place for her in the darkness of his head.
The kids from Freaks and Geeks were college students who spent a few days visiting while doing a film project on grief
Anna, Ruby, and Jo were all patients.
Meg and Garth were part of the nursing staff, and eventually moved onto other jobs.
The angels were doctors - the good and the bad.
Charlie was visiting her mom and read to those who never seemed to have visitors, including Dean when Sam wasn’t around.
Sam caught her one day and they chatted and became friends, Dean sometimes overhearing their conversations and imagining Charlie on adventures with them.
After a while, she became an activist for proper care for patients that weren’t receiving it (including her dying mother), i.e… “Save the patients.”
When her mom finally passed, she came in to read to Dean one last time - the Wizard of Oz. When they finished reading and said goodbye for good, Dean pictured an affectionate final exit.
The Ghostfacers were goony, new age paranormal investigators who snuck around posing as med students while checking out the hospital, believing it was haunted because of its history with 18th century epidemics. They got repeatedly kicked out.
Crowley was an investigative journalist who reported on major accidents and was responsible for interviewing victims. His lack of privacy and insensitive comments to the families while conducting interviews gave him a demon’s image in Dean’s mind.
After years of work, he took an interest in Dean and occasionally entered his room out of curiosity. His speeches about demon deals were really him thinking aloud about the stupidity of some of the poor souls he was writing about. Eventually, he left journalism and to accept a job grant writing for the research units at the hospital, becoming a regular.
And after all this time, Sam’s still the only one who still visits loyally.
Demons and talking monsters are the nursing assistances that are too rough or neglectful, and make cruel comments. Hell hounds are visitors that get too rowdy and start screaming in the halls outside his room. The monsters and all the things Dean is so hell-bent on fighting are his own demons - things he imagines in place of all the things he can’t deal with, like a scared little kid that never really grew up.
Supernatural is just a bad dream for a man that couldn’t wake up.
IT ALL MAKES SO MUCH SENSE NOW