Dean Winchester and the vampire Benny became brothers in arms in Purgatory. At first meeting, Dean didn't trust Benny despite the fact that Benny saved his life. Together, they begin to fight their way out. Upon reaching the portal out, Dean keeps his bargain and takes Benny's soul with him back to earth. Outside, they agree to go their separate ways, and that Benny should keep his nose clean to stay out of trouble.
Benny calls Dean for help and he helps him take down his old nest. While Dean initially distrusted Benny, he grew to trust him absolutely at some point, presumably after he saves Castiel from a Leviathan despite his dislike for him. By the time they are ready to escape Purgatory, Dean tells Benny that he has earned a lot of trust from him. At one point he tells Sam that Benny is the only person who has never let him down. Dean's trust in Benny goes so far that he defends Benny when it looks like he fed on humans, believing his story even when he catches him red-handed burying another victim about it being another vampire. He goes so far as to send a fake text from Amelia Richardson to Sam to get him out of the way and defends Benny's killing of Martin Creaser as self-defense, knowing that the mentally-unstable Martin had been after Benny and having gotten the story from his great-granddaughter. However, when Sam made Dean choose between them, Dean ultimately chose Sam over Benny and cut all ties with him, something Benny was understanding of despite his deteriorating situation.
Learning that Sam is trapped in Purgatory, Dean calls Benny to rescue him. Dean's plan is to kill Benny to send him back so he can lead Sam and Bobby out the same way he did Dean and come back with Sam. Dean knows that he is asking a lot, but Benny agrees without hesitation, telling Dean he doesn't fit on Earth anymore. Benny agrees to come back with Sam, but secretly plans not to return, keeping this from Dean. Dean kills Benny who rescues Sam and Bobby as planned, but stays behind to hold off other vampires. Dean is devastated to learn that Benny didn't come back with Sam and buries his body instead of burning it so he can possibly bring Benny back in the future. Benny's sacrifice earns him Sam's trust and he agrees to Dean's action, ending the animosity between Sam and Dean about Benny.
The Werther Project
When under the influence of the Werther Box, Dean hallucinates that he is back in Purgatory with Benny who he is glad to see until he realizes that Benny isn't real. Dean kills his hallucination of Benny to return to the real world, though he does this knowing full well that it is not his friend that he is killing.
Lady Toni Bevell attempts to question Dean about his relationship with Benny. Dean is obviously startled to have Benny brought up again but doesn't answer her questions.
Fans began writing this pairing the day of Benny's appearance in 8.01 We Need to Talk About Kevin, spurred on by the enthusiastic hug between him and Dean.
Within a couple of days, a community based around the pairing - as either a romance or bromance - was started on LiveJournal called USS Denny.