Disclaimer: I don't own any of these characters, or the show.
I.
the first time Dean kissed Castiel was in the Impala. In retrospect, it wasn't much of a story—there was no great realization, no heart-wrenching 'I love this man and I need to show him'. Dean didn't give it any thought, it was as natural as breathing when you come up for air after swimming, when the water runs into your nose and mouth and it's uncomfortable, at first, but you know you need this, it's vital, and damn consequences.
It was just like that.
Dean was bleeding and hurt (and damaged and lost and stoic, in that way he had drilled into himself for so long) and Castiel was unharmed, outside, but where his flesh wasn't un-knit and bruised his heart was, because he was supposed to be a soldier and not a warrior, and he hadn't realized yet how similar/different the two were, but some instinctive part of him said, it's different.
Green eyes met blue eyes after the first impact of lips on lips, and that was the first time then that it was considered, after. And even then Dean didn't think, didn't let himself think or freak out because a part of him always knew that there was a profound bond, and damn if he knew what that meant but he'd take it in any way, shape, or form it was offered.
One kiss became two, the sloppy—real—honest kind, with bumped noses and hovering hands and teeth get in the way. It was something tangible in a turbulent world, a real thing in a dream, a moment when you know something is fact even among lies.
II.
the first time Castiel kissed Dean was much, much later. Dean looked up from the map he was marking on to find a hovering ex-angel, staring at him so intently he found himself squirming, like he was being pinned under the sheer force of one set of eyes. Then a pair of lips hit his, catching him by surprise, but that didn't last, oh no, because Dean Winchester was not one to be taken by surprise and he kissed back, hard.
Whose hand shoved the map of the table wasn't ever really known but the cleared area was rapidly replaced with a trench-coat wrapped man and a couple of roaming hands that didn't belong to him.
This time there was thought, there was planning, because Cas isn't Dean, oh no, and he needs to understand why because if something doesn't make sense it can't be taken at face value. Why he had never kissed Dean before then, in the few weeks that spanned the divide between the first time Dean had kissed him and the present, wasn't clear in the way it should have been. It was filled with 'I don't knows' but that moment he had seen Dean leaning over the map and Sam was out doing Sam things that still weren't really clear to him and Dean had look haggard, and worn, like the jacket tossed over the back of the chair and Cas had seen, truly seen the life of Dean Winchester carved into every scar and his crow's feet and the dry skin of his knuckles, and Dean looked happy, like everything he ever wanted was spread out on that sheet of paper in the next destination.
If that's not a person to kiss, to savor, then Cas doesn't know what one is.
III.
the first time Cas said 'I love you', Dean wasn't even awake. Dean still thinks he said it first, because Castiel never told him.
The thing, the horrible truth of it is, Cas understood the word much better than Dean. Love is such a big, big word, and this was even before Cas understood what romantic love was, let alone sexual attraction, and he knew. He knew he loved Dean Winchester from so early, but he didn't have a name for it, because he heard 'love', but it was a trivial word, a soft one. it wasn't the all-encompassing need, the feeling that your heart and soul is wrapped up in one person, and it's both okay and not okay because on the one hand this is something you can't go back from, loving someone, but on the other if Cas couldn't love Dean, couldn't trust Dean with his love and affection and his very breath, then Cas couldn't give that to anyone.
And Dean was wounded—not badly, not as badly as he'd had, not even troubling, by his standards—and he was asleep, trying to sleep through the ow, hurt, something's wrong feeling. Cas had stood in the back of the room and gazed at Dean as if Dean was the only thing in the room, the only thing in the world, because to Cas he was; he was an eclipse in the span that was Cas' horizon and everything that was once big was little. So Cas had put his hand over the imprint on Dean's shoulder and leaned down to where his lips brushed the curve of Dean's ear and he whispered it like a sacred promise, like it was solemn oath, that Castiel loved Dean and never would that not be true.
IV.
the first time Dean told Cas he loved him was kind of a big deal, I mean, it wasn't something he said lightly when it was actually gospel. He knew that the push of his lungs outward, the beat of his heart in a constant 'now now now' and the step, step, step of his feet could be halted at any time, frozen indefinetly, and all of a sudden he couldn't bear the idea of never having said it.
"I love you." He had stopped, and Cas had turned around and tilted his head at Dean in that endearing, infuriating way. Dean stepped forward and shook his head when Cas opened his mouth to respond, maybe reciprocate, but Dean didn't know because he pushed his mouth over Cas's and pushed a tongue between Cas's lips because he didn't want Castiel to answer then. He did not need an answer then. He knew.
V.
the first time they had sex was awkward in the way sex is always awkward, which is something nobody says and everyone knows, compounded by the fact that Cas was clueless as to everything the pizza man didn't cover, which was actually a lot of territory that Dean would have to cover, both literally and figuratively, and soon.
In the meantime there were a lot of elbows and muttered apologies, and a lot of missing when you tried to kiss and ended up pressing a kiss on a cheekbone or forehead. There was a lot of promises and begging and 'please', 'yes', 'oh's, and there was almost a few 'oh, God's' or 'Jesus Christ's' but Castiel managed to plant a mouth or a hand over Dean's mouth before he had to deal with hearing those names when he was worshipping someone else.
They picked up on each other's likes and dislikes and 'never, never again's, and sheets got pushed down and over until they fell in a swath on the floor around the bed because they were in the way, damn it, and seriously Cas you just try to leave now to grab those and I swear…
one.
they forgot the time they told Sam about them. It was quick, of course, as Sam was more than family if there's anything more than family, and he would have figured out soon enough anyway. They forgot how awkward it was, trying to explain why when they didn't know why, and Sam didn't bother telling them that he knew why. He didn't have a word for it, no label to sum up. It was a knowledge gained from seeing shared glances and lingering touches and the way their eyes flitted to each other after something bad happened, as if to verify and assure themselves that their other half was safe and not bleeding out. Plus, they eye-fucked like, everywhere, and really guys, get. a. room.
two.
they forgot the first time Dean used the word 'soulmate' in reference to Cas. Cas thinks it, all the time, uses it as an interchangeable pronoun for 'Dean', but Dean? Not as much. It wasn't until after he said it, just talking about Cas casually in conversation, that he realized that he did just use that word for that person, and when he did he took it in stride, like, 'huh, how 'bout that'. What else would he do?
three.
The first time one of them got jealous, it was over nothing. Surprisingly, it was Cas, not Dean. Dean had gone to a bar, which he did occasionally, on those nights when everyone did their own thing and there was nothing to kill. Some girl had been flirting with him, wanting a beer, and while he bought her one he didn't flirt back. In the end she had physically thrown himself at him, planting a kiss on him before he could react, and she stumbled off to that bathroom to throw up in what Dean hoped wasn't a personal remark on him and his kissing skill. It would just be his luck that his car wouldn't start, so he would have to walk the way back to the motel they were currently residing in. That was the reason, the real, only, reason that he stumbled back much later than he had promised, smelling of cheap perfume, with lipstick streaked across his cheek were he couldn't wipe it off.
Try telling that to an ex-angel of the lord, vanquisher of armies, in sweatpants and naught else, frustrated and terrified that his Dean was hurt, only to have him stumble in like this.
But no, he didn't stay there and hash it out. It's Cas's curse that when it comes to this, here, his fight-or-flight instinct is more heavily weighed to flight. Thus, Dean found himself locked out of the bathroom, trying to shout explanations through the solid wood door while he could hear half-gasps from inside and nothing else, as if Cas was just sitting there trying to believe Dean. It didn't even occur to Dean until late, much later, that Cas had closed himself in that so that he would have the bed, and when he realized that he pulled Castiel that much closer and pressed his forehead to his spine, throwing a leg over his legs so that there was no escape, not even in sleep. As if Cas would have gone.
They forgot it in the months later, when old fights gave way to new ones. Another lost moment, blissfully forgotten, possibly for the better.
four.
The time Dean found out Cas was ticklish was another time they forgot. Dean never forgot that Cas would wiggle and giggle if you brushed your hands there, just there, along his sides, from his ribs down to the tops of his thighs.
And the funny thing (besides the squirming, laughing angel), is that Dean wasn't even trying. It was a lazy morning, which were rare and valuable, like a piece of gold, and they had woken and soft 'hellos' were exchanged along with soft, wet kisses, morning breath ignored. Kisses had morphed into touches into rolling, arching movements in that languid, unconcerned way, and gasps and moans and one sudden giggle.
"Seriously?" Dean had muttered gleefully, pulling his head back from his partner's ear and sliding his right hand up to join his left, at Cas's abdomen. "You're ticklish?"
"I do not know." Cas said, attempting something like solemnity but failing, miserably, because he had to slide his hands from Dean's chest to Dean's wrists to get him to stop, while he snickered and writhed. "Get back to what you were doing." He distracted the green eyed man by guiding his hands back to their previous positions and snaking in for another kiss.
"Yes, sir. But don't think I'm forgetting that." He warned, against Cas's mouth, and laughed. He never did forget that Cas was ticklish, even if he did forget how he found that out.
1.
never did Dean doubt that Castiel would stay with him, forever. It was an accepted law. It would be written, wherever laws are written, like 'for every action there is an equal and opposite reaction' and 'the earth is not the center of the universe'. If Cas wasn't there in body, he was there in spirit, in heart. There was never a moment, from the first time Cas had decided it would be so, that Cas left Dean entirely. Never did he abandon him.
2.
never did Dean not need Cas. Like air, like blood, like the feel of the road under his feet—it was a boon, a blessing, that the two went hand in hand, because if Cas ever left him entirely Dean would have faded, dragged to a slow stop, because he's needed Cas more than even he knows. From the first time Dean's soul had said 'yes, that's what we've been looking for' or however it is that souls declare such things, Dean has needed Cas like nothing else. He needed his brother, they were a matched set designed to go together, but it was Cas that made him complete on his own, kept his part of the set from crumbling under the sheer wrongness of it all.
3.
never did one have to live without the other. It was each's fear that the other would die first, leaving them, broken pieces of a barely-breathing story, to face the monsters in the closet and under the bed.
It didn't go down like that. It must have been the will of those two men that not even fate could hold out under. They never made it to a ripe old age—it wasn't necessary. They lived enough in those years they had, saw enough disaster and miracles to fill a dozen lifetimes, especially taking into account the fact that Cas had already seen so, so much.
A blaze of glory, they say. It was a story Sam told his kids and they told theirs, about the last battle and the gunfight and the two brave men who didn't make it out but made sure all those others did.
Together they departed for wherever the hell those two would go, after, and it's a safe bet that even then they weren't separated. Heaven and hell had already had a go at tearing them apart, and been put in their places—what's the worst they're gonna do, try it again?
They could never succeed.