Shaking up Winchester World
Cas accepts everything Dean does. Not without question, but at least without vocalising those doubts. Which is more than Dean can say for Sam.
But it's harder to argue with someone when they disagree without words. Sam calls it 'passive-aggressive'.
Although Dean has gotten used to Sam's Puppy-Eyes™, he has yet to develop an immunity to Cas' Disappointment Face™. Dean thinks his Disappointment Face looks a lot like someone has pushed over Cas' granny so they could steal her walking frame and beat her seeing-eye puppy with it. Specifically, as if Dean had done the pushing. And the stealing. And the puppy-beating.
He is the first to admit that Cas' Disappointment Face leads directly to Dean sighing loudly and saying "Fine." Which then leads to one of Cas' Miniature Smiles, which usually leads to one of Dean's (totally not) Secret Smiles, which usually leads to Sam's trademark bitching and on a good day (well, according to Dean), a separate motel room for Sammy. The circle of life continues and all is well with the world.
Dean can't explain it exactly, but it's good to have someone expect more from him. Like if Cas thinks he can do better, then maybe, he starts to think he can do better.
Sam stopped expecting Dean to change a long time ago.
Cas asks Dean questions that no one has ever asked him before. It makes him think, and it makes him re-assess what he thinks.
Dean is used to questioning authority. Cas takes it one step further and questions everything. It reminds Dean of when he and Sam were kids and everything always came down to "Why?"
Sometimes Cas' weird-ass, uncomprehending view of the world is frustrating:
"Pass the socket wrench."
"Which one is the socket wrench?"
"A wrench. With a socket end…Never mind."
"You are not wearing those runners."
"I can see that Dean."
Other times it's surprisingly seamless:
"Sam! We need a banishing circle!"
"Did you bring spray paint? A permanent marker? Anything? "
"No. Did you?"
"Maybe we could use your lipstick, Sam," Dean says with great facetiousness.
"Bite me. You weren't exactly prepared."
Cas has been watching them like a table-tennis match. He looks at the nearest kitchen object, the refrigerator, and opens it.
Dean tells him off. "Now is not the time to get the munchies, Cas."
Cas, as usual, pretends not to hear Dean. He knows Dean is just redirecting his anger with himself for being unprepared for a simple domestic haunting. Cas thinks of all the times he has seen Dean stand in front of a fridge and pull out pie and cream.
Cas searches the refrigerator shelves, reaches in and tosses Dean an aerosol can.
"Cheese Easy? … You sure there's no Dairy Whip?" Dean shrugs and looks at Sam who raises an eyebrow of appreciation. "It'll do." Dean draws the circle with Cheese Easy.
There is Dairy Whip, but Cas is saving it for later.
Sometimes it's good to have a fresh point of view in Winchester World.