Case fic idea: Bad Mood Sam
One day after a case at a rickety old haunted house, Sam starts being extra MEAN about everything. It starts to get a bit overboard:
"I've spent the last three days assuming everyone I meet is an idiot."
"I don't usually say it tho."
It escalates. Sam realizes, "I need help," and Rowena is like, "I don't see anything wrong with you?"
Dean is like: "Jeez, Maybe some Prozac?"
Mary has left the building, and she took Jack with her.
The only reason Cas is still here, is because he's semi-shielding Dean from Sam's epic bad mood. (The argue about coffee, about lore, about everything.)
In the end, it's a supernatural splinter (from the haunted house) that his skin healed over and hid from view. Dean tries to remove it with tweezer but is unsuccessful.
So Cas takes delight in digging it viciously out of his arm. (Like I said: Sam's been MEAN.)
After it's all said and done, Sam is still a little meaner than before because some parts of it he found empowering.