If there's one thing Cas has learned about Sam and Dean Winchester, it's that they don't talk about it.
They don't talk about how they both know what it's like to have to bury your own brother. What it feels like to watch the person you care about most die in your arms, and then see them return, smiling, just a few months later.
They don't talk about everyone they've lost. Or how they've both experienced the soul crushing heart break of seeing the love of your life die in a burst of light, again and again.
They don't talk about what's in each other's heavens, or what's used to torture them in each other's hells.
And they certainly don't talk about what's missing in heaven, and always present in hell.
Cas often wonders if there's a list of everything the boys won't talk about. He's tried to make one himself, but he knows there's always things he's forgetting.
They don't talk about Gabriel. Or Lisa.
Or anyone that has vanished from their lives in one way or another.
They never mention John. Not anymore.
They never talk about how it feels to stick a shovel in the dirt of a field, knowing that the hole you're digging is the one you'll be leaving your sibling in.
And no matter how many times the boys die, it never gets easier for the other one to deal with. They joke about it all they want, but they both know deep down that every death only breaks them a little further.
They don't talk about the number of times Dean has come dangerously close to killing himself.
They don't talk about the antidepressants Sam used to talk, before even they became not enough to help him.
They don't talk about the nightmares, the demons that haunt them.
Cas finds it interesting how two people who are so close refuse to share so much.
He thinks it's interesting that he knows all of Dean's pain, and Sam knows all of Dean's happiness.
Because Cas saw the pain. Every awful thing the elder Winchester had gone through, internally and externally, emotional and physical. He saw Dean Winchester's true form when he pulled him out of hell, and it was difficult to recognize the man under all the scars.
But Sam doesn't know that. Sam knows some of it, yes. Some of hell because he's lived through it side by side with his big brother. But he doesn't know the half of Dean's struggles.
But he knows the joy. The happiness. Because nearly every moment of happiness in Dean's life has to do with Sam.
And nearly every happy memory Sam can think of has to do with Dean.
That's why they share a heaven. It seemed pointless to put two people with the same set of happy memories in different places.
That's why they always save each other. Because when one dies, it's not just their brother that's dead. It's their happiness too. Their will to live, their reason to wake up in the morning.
If Sam and Dean didn't have each other, then they wouldn't have anything at all.
But they don't talk about that.
They don't have too.
They know.
Dean knows not to talk about Gabriel. He sees the way his brother winces, how his eyes go dark with regret and sadness.
Sam knows not to talk about Cas. He sees the way his brother tries to fight to keep the revealing smile off his face. How he'll deny anything even though his eyes beg to tell everything.
They both know better then to talk about anyone they've lost. It only hurts them both.
They know the touchy subjects to avoid because half the time, they don't want to talk about them either.
But sometimes they need to talk about it. And Sam tries. Tries because he knows, and he wants Dean to know that he knows, and to let Dean know that's it's okay. It's okay to have things you want to avoid, but it's also okay to talk about them.
Dean only tries when he knows something wrong with Sam. If something's off, and his brother needs to know that Dean's there, and no matter how much he hates the chick flick feeling crap, he'll put up with it if that's what Sam needs. But not if he needs it himself. Never.
That's how he was trained. Show no weakness, have no weakness.
But Dean does have a weakness. And so does Sam.
Their weakness and their strength is each other. They need each other more then anybody else, because no matter who else comes and goes in their broken little family, the only ones they know will always be there are each other.
But they don't talk about that either.