Dean:
You met at a bar. Typical of course. But you owned the bar, he was a regular because of where he lived. He came down every night even when he wasn't going to drink anything he came to keep you company.

Sam:
He was on a hunt and was at the local library, he accidentally bumped into you in one of the isles. When you asked what he was looking for he told you the truth and guessed he was a hunter so you showed him all the book on the supernatural lore.

Cas:
You were with Dean when he went to meet the person who pulled him from Hell, you met Castiel that way and ended up praying to the Angels more often.

Crowley:
You were a measly demon who had never met the King of hell. When you did he didn't act the same as you had heard.

Lucifer:
You met during the apocalypse, you were with the Winchesters and he took a liking to you, so he tried to talk to you through many different ways including possession.