You never really say it to him.
Goodbye is a word avoided on principle, because truly any day could be your last. Bad luck has a way of following you around. Of course, that doesn't mean there aren't times to say goodbye. You don't. You don't want to think about it, and so, by the sheer force of your Winchester-bred will, you don't. It's a weak, stupid word said by weak, stupid people, you tell yourself. When did you become so good at lying?
Since forever, actually. But you could never lie to Castiel.
When you want to say it, you just meet him.
He's an angel, and you can't think about that without your head hurting. Castiel pulled you out of Hell, somewhere in your soul you can feel it. The problem with that isn't that you escaped, because you would kiss him for that, but...if God is up there for real, wouldn't he care just a little?
He cared enough to send Castiel to get you.
No, he didn't. Castiel came with a bunch of other angels to kill demons. You were just something extra.
But he found you and he saved you. He may have a stick up his ass, but Castiel is profoundly good. You can barely look at him without feeling it.
You're scared that you won't be enough for him, that he'll say he made a mistake and throw you back, but you don't say goodbye.
When you want to say it, it's Halloween.
Sam is struggling with Samhain and has demon blood pumping through his veins, but you two save an entire town. Castiel and Uriel were going to burn it to the ground and start over, so why is it so hard to justify your motives to him? Maybe it has something to do with his eyes. His blue, blue eyes that question you when you don't want to be questioned and plead with you when you can't say no.
Those eyes also hold belief in you. On the benches at the playground, Castiel tells you that he doesn't know whether you made the right decision in the long run, but saving people will always matter. He makes sure you can't look away when he speaks, and you don't know how, and you don't want to.
The world is so small when you look at it through him, but it's a beautiful world that he thinks you can save.
You don't want him to follow you and get hurt, no matter how irrational that may seem, but you don't say goodbye.
When you want to say it, you're about to confront Lilith at the little church in the middle of nowhere.
Sammy and Ruby are too far gone, you think, but you still believe you can pull Sam back. Right now, though, you need him hopped up on demon mojo because Lilith can't hurt anybody else if she's dead. You hate it, and you hate that he can't be there to help you.
Not that Castiel, angel of the Lord and kicker of demon ass, has anything better to do. (It shouldn't sting this much. You shouldn't be this dependent.)
You think about praying to him before you get in. You don't. Instead, you keep your head high and go in swinging, the way you always have. It should feel good, being reckless and too self-sacrificing for your own good, after all, it's always helped before. Now, all you want is to get it over with and leave. You want to drink a beer and screw the Apocalypse. You want him whispering in your ear that he will be there when you get back.
Of course, you can't ever get what you truly want, especially not from an angel.
The skies are dark when you walk in, Sam and Ruby close behind you, and you wish for one fatal second that this isn't all that you will be: a hunter that failed one too many times.
You want your legacy to be more than the man who died for nothing, but you don't say goodbye.
When you want to say it, the world has become a foreign place.
The future is ugly and so are you, five years later. The possibility that you could kill so many innocents isn't exactly a confidence booster, and neither is the fact that the first person you want to talk to here is as high as a kite.
Cas is human, painfully so, and broken beyond repair. All you want right now (besides getting home) is to fix him, but you can't. Zachariah is a sick sonovabitch to screw with him like this, and you would like nothing better than to beat him to a pulp. But you can't. He would find some way to hurt Castiel again, and you couldn't do that to your angel. You never ever could.
When he's high and driving you to the meeting place, you ask Cas how he got this way. "I lost my angel mojo, and so, after becoming human, I just couldn't take it. You went nuts!" He laughs. A spike is driven through your heart.
"But I can fix this, can't I?" Your voice is desperate, and you wonder if he can hear it. Maybe he can't, you reassure yourself.
"Honestly, I don't think it matters. You're going bad, whichever way you choose, Michael, or no Michael." He laughs again, but there are tears in his eyes. "I'm sorry it had to be this way."
"It will never be this way. I promise you, it will never get this bad as long as I'm alive." Cas touches a finger to your nose and smiles.
"Thank you, Dean. That means a lot."
You want to be able to fix everything, and the idea of not being able to do that is unthinkable, but you don't say goodbye.
When you want to say it, your very last day could be tomorrow morning.
You, Sammy, Bobby, Ellen, Jo, and Castiel are standing around Bobby's living room, beers in hand, not saying anything. You want to mark this occasion with some sort of celebration, after all, that's what normal people would do. Last night on Earth and all that; it only seems right. Then again, you left 'right' behind a long time ago.
"We should take a photo of all of us," Jo pipes up once the silence has started to crush you all.
"Good idea," Sam says, jumping on the bandwagon. You just want this night to be over, you want the day to come so everything can happen, either outcome is welcome. But you could never deny Sam anything (or Castiel, but he's not a part of this) and so you gather everyone around.
The camera is set on a timer so that no one has to be the cameraman, and you're glad for that, because you want just one whole, complete memento from your hunting days. You wonder if you could possibly stop hunting someday, but you toss that idea in the trashcan. Metaphorically. No matter how much you don't want to lose what you have, you want to save people more. Saving Sammy ended in a giant pile of shit for the both of you, and you can't afford to do that again.
Cas gives you a look once the camera flashes and everyone can finally move from their positions. You follow him into the kitchen and ask him what he needs.
He asks, "Can I tell you something in complete confidence?"
You say of course he can.
Castiel takes a deep breath. "I'm frightened of losing any one of you tomorrow. That has never happened before, not when I was fully angelic, and I want to know what to do about it."
You smile at him, even though it's super inappropriate and you shouldn't. "You hold onto what you have until you can't anymore. You make the most of this and the rest of us will make the most of this, and when we come out of this mess, we'll be the most together. You got that?"
Cas nods. "That makes sense. Thank you, Dean." He quickly wraps his arms around your waist, still awkwardly holding his beer, and then lets go, heading back into the living room. You don't go after him for several minutes, trying to process what just happened.
You are terrified of losing him. What you want more than anything in the world is to say goodbye to him, just in case you die, or he dies, and you never tell him all the things you haven't, but you don't say goodbye.
When you want to say it, your brothers are having it out in a cemetery. Castiel followed you here, you know he did, and Michael and Lucifer are using your little brothers as punching bags. You hate both of those damn angels, but if Cas is on your side, it really doesn't matter.
You two fight as hard as you can, which means you end up beaten into a pulp and Cas dies (you see the outlines of his wings on the ground). It's so painful right now; literally everywhere on your body hurts, especially your heart. You want to die so much, just to get out of here, but you'll end up back in Hell with no angel to pull you back out, so is wanting to die really worth it? The goodbye steams on your lips like boiling water.
Everything after that is fuzzy; Sam and Adam end up in Lucifer's cage, you know that, but the exact how and when of the event is unknown. All you can feel right now is fire.
Suddenly, it's all clear again. Light.
And he's looking at you again. Blue, blue eyes.
"Cas? Are we in Heaven?" you ask.
He smiles at you. "No, Dean. We're alive."
...
When you drive away from the graveyard, Cas sits in the passenger seat, silent. He knows that talking wouldn't help anything, and it comforts you. He knows you. "Where do you want to go?" you ask him. Sam wanted you to move on from the hunting life, he told you so, and now, you are.
"Well, I haven't been to New York City in a long time." Cas looks back at you with a small grin gracing his face.
...
Four hundred miles later, you abruptly stop the Impala. He wonders why, you both are far away from a gas station or any sort of civilization. "I never want to say goodbye to you," you tell him, because it took you this damn long to figure it out.
Cas reaches across the car and kisses you on the cheek. "I never want to say goodbye to you either."
And so, you never do. You said goodbye to Sam, to Ellen and Jo, to your dad, to your mom. But now isn't the time for that.
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