Pretty much every rock and metal band in the history of ever had at least one song on at least one album which was a slow ode to love lost, or love found... Just love in general. Sam knew they were Dean's guilty pleasure; he listened to them when he thought Sam was out of the house. Sam thought he'd stumbled upon one of those moments where he catches Dean singing along and mocks him about it until he accidentally does something worse and the mockery turns on to him. What he'd stumbled upon instead was definitely not what he was expecting. He couldn't stop watching. It was like a car crash. He knew he should have turned and walked in the opposite direction, but he couldn't tear his eyes from the sight of his brother and his angel-boyfriend slow-dancing in the middle of the kitchen; their bodies pressed together, faces buried in one another's necks. They slowly shuffled around in wide circles like an awkward high-school couple at a dance, but with very fluid, synchronised movements. Cas had a hand clasped on Dean's shoulder. The shoulder. His other raised in Dean's hand. Dean's arm was wrapped tightly around his lover's waist, holding him in place to stop him leaving.
They'd already had that argument. About Castiel leaving. It was actually the argument that spurred the two ignorant, "I wouldn't see your burning love for me even if it stabbed me in my aching heart" men to get together in the first place.
Dean had seemingly forgotten his younger brother was in the bathroom of their skeezy motel-of-the-week and could hear everything. He could hear the loaded statements laced with hidden meaning come from Dean; "everyone ends up leaving me," translates in to: "please stay with me." "Sure, Sam's there," meant: "I can't curl up in bed with Sam and feel warm and whole." "I just don't want to be alone," also meant: "I need you to stay. I need you."
But of course, Dean-emotionally constipated-Winchester couldn't say any of that. Instead he shouted, and swore, and from what Sam heard, came very close to throwing things at the poor angel.
Castiel, for all his lack of understanding of human behaviour, was able to read Dean very well, and pressed him to ask him to stay. He urged him to say those four single-syllable words, and he'd stay.
However, verbally acknowledging he wanted Castiel to stay with him was as easy for him to utter as "pie sucks." He couldn't do it.
"It's implied, Cas! Can't you just...?"
"No, Dean. I have nothing to agree to. You need to ask me to stay!"
"I am asking you!"
"You need to say it! Out loud! Four words, Dean. And I'll stay!"
"For how long?"
"Forever, Dean. I'm yours if you want me, I think you've always known... You just need to ask me."
Dean sighed loudly; "Cas..."
"Then neither can I."
"-A demon doesn't make a deal without a clear contract. An angel can't make a vow if it isn't wanted of him. You need. To ask."
"Liking yourself to demons now, Cas. Classy."
"Don't deflect, Dean! I want to stay, but I have duties."
"Then go! Don't let me hold you back! You've got shit to do! Go do it!
There was a sigh and a flutter of wings. Sam sat by the bathroom door, wondering how long was long enough until he could go back in. He gave it five minutes, clicked the lock open and slipped through the door. Dean was lying on his bed, his arm thrown over his eyes as he took slow, deep breaths.
"-Don't, Sammy. Just don't."
Sam nodded, shoving his hands in to his jeans pockets; "do you want me to go grab some food? We can head up to Bobby's in the morning, stay for a while. Recharge?"
The elder Winchester quirked a sad smile; "sure. Sounds great."
The two hunters left for Bobby's that night after eating instead; packing everything in to the Imapala and starting the long drive. Dean chose to take the back-seat, citing he'd earned himself a long nap, thrusting the keys in to Sam's had and issuing the standard warning.
The next day, after the hunters had made it to Bobby's and had some sleep and food, Dean laid claim to the kitchen for several hours, armed with a coffee machine, bottle of whiskey, and a notepad and pen. Sam had heard Dean muttering something about Cas getting a contract as he rooted for a pen that worked.
"What's eating him?" Bobby grunted, popping the cap from his beer. Sam shook his head, popping the cap of his own.
"Denial? Self-loathing? The fact he's an inch from being happy and he can't say four words to the love of his life?"
"That damn angel."
"To be fair, Cas gave Dean everything he needed, Dean just needs to ask him to stay, but he can't."
Bobby frowned; "so what's he doing now?"
"Writing a love letter?"
"Will that work?"
"I hope so! Dean can't hunt like this."
After his commandeering of the kitchen, Dean barged out of the front door, shouting something about grabbing food and sped off down the drive. Sam cautiously made his way to the kitchen, stopping when he heard the flutter of wings. Peering around the door-frame, he looked in to see Cas pick up an envelope from the table, peering in at it's contents. He tipped it out on to his palm. It was a ring. One of Dean's rings. Holding it between his thumb and finger, he analysed it, his brows knitting together, before setting it down on the table and plucking the letter from inside the envelope. After a few minutes of staring blankly at the paper, he looked up, his eyes instantly meeting Sam's. Sam blanched, afraid he was going to be chastised for spying. He wasn't expecting the shy smile from Cas he returned instead.
"Sam? Could you come here?"
Nodding nervously, Sam shuffled over.
"I'd like you to call Dean, please. Tell him to come back."
"He's just left!"
Sam pulled his phone from his pocket, flipping it open; "all right... What do you want me to tell him?"
"To come back. Please don't tell him I'm here. If he asks, I've left."
He quickly pulled his brother's number up, pressed call, and waited for Dean to answer. Castiel sat down, playing with Dean's ring. The call finally went through.
"Dean, can you come back?"
"Just... Come back?"
"...Is Cas there?"
"No. He took off. Grabbed your letter or whatever and left."
"Just come back and talk about it, man. Maybe we can help?"
"-No, Dean. Just come back! Now!"
Dean hung up; Sam looked from his phone to the angel, currently twirling the silver band around a finger.
"Thank you, Sam."
Sam slumped down in to the chair next to Cas', picking up the letter and looking over it. It looked like Dean had a whole speech planned out, but had scribbled it out at the last minute. He could still make out phrases and sentences; "I love you," "I don't think I'll be able to function properly without you," "just stay you stupid bastard," etc. In the end he'd just written five words.
Please stay with me, forever.
Sam couldn't help but snort at how surprisingly pathetic his older brother was. That was also the moment he realised how deep Castiel's love for his brother was.
"Any being in existence, and you chose Dean."
Cas smiled meekly, twisting the ring from his forefinger and slipping it on to the middle finger of the opposite hand; "we don't choose who we fall in love with. I'm sure Dean would prefer to not love me, but he does... But I understand what you're trying to convey; yes. I'll always choose Dean." He began looking up at the ceiling reverently; "I'd fall a thousand times for him; unmake the world for him. I'm helpless, Sam, I don't think there's anything anyone can do... Do you understand?"
Sam smiled, looking down at the letter in his hands; "yeah... It hurts. A lot. But you don't want to feel any other way."
"...Are you going to fall? Because of this?"
Castiel paused before shaking his head; "I shouldn't. After all the things I've done up until now and I haven't fallen yet. I've been brought back again and again..."
"Well... What if Dean is the last straw? You know... Having feelings for a human."
"Feelings aren't a problem, Sam. I'm here, I'm still an angel, and I have strong feelings for your brother. Do you mean a physical acknowledgement of our love?"
Sam winced, his mind burning with images of Dean and Cas doing things he didn't particularly want to imagine; "probably."
"No. Angels and humans have copulated long before today, and potentially will long in to the future. The only 'abominations' were the nephilim, and neither Dean nor myself can carry a child, so there is no need to worry."
"Well... Good. No, seriously. Awesome. It's about time. Dean's going to kick himself when he realises how easy it really is. All the time you two have missed out on. Him and his stupid macho attitude."
Cas smiled, pulling himself to his feet; "I wouldn't change a thing about him."
"Of course you wouldn't."
The familiar rumble of the Impala's engine sounded outside, suddenly cutting out. Sam pulled himself to his feet too, handing Castiel the letter, going to make himself scarce when the back door opened and Dean stomped in. He stopped in his tracks when he noticed the trench coat wearing angel stood in front of him. A mess of emotions spread across his face, ranging from relief that Cas hadn't bolted, to apprehension that he had to face Cas, to anger that Sam had lied.
"-Cas asked me to. Anything to get you here. You two need to talk. I'll-"
"-You can stay, Sam. We don't need to talk," Castiel interrupted, taking the several steps closer to Dean, pushing the crumpled letter in to the hunter's hand. Dean tongue quickly slipped out to wet his lips, his eyes fixed on the angel's.
"I'd have preferred a verbal request, but I'll accept a written one."
Castiel took Dean's hand, facing his palm upwards, placing the ring in the middle. Dean frowned.
"That's for you," Dean almost-whispered. Castiel still smiled, nodding as he held his left hand out in front of him.
"I know. I'd rather you give it to me in person."
Dean chuckled, his eyes beginning to sting as they teared up; "this isn't a proposal, Cas."
"Perhaps not a marriage proposal, no. But it is still a proposal, of my staying by your side."
Sam could feel his cheeks burn as he watched the most heartbreakingly romantic thing he'd ever seen play out in front of him. Dean slipped the ring on to Cas' ring finger, a wobbly smile playing on his lips.
Cas cupped Dean's cheek, softly stroking the skin with his thumb; "forever."
Had Sam blinked, he would have missed Dean surge forwards, crushing his lips to Cas'. The youngest Winchester smiled, happy for his brother and Castiel as he slipped out of the kitchen, muttering about not needing to see them start making out. He wandered back over to the front porch where Bobby was still sitting in the same place, still on the same beer.
"Saw Dean came back. I've just missed something, haven't I?"
Sam chuckled, helping himself to another beer; "I think Dean and Cas just got angel-married."
Bobby raised his beer in a sort of toast; "Mazel tov."
Sam laughed clinking his bottle against Bobby's, taking a hefty swig.
Dean and Castiel were nearly inseparable from that point onwards, literally. The two were always touching each other in some way. Hands clasped together, thighs pressed together, a hand on a leg, an arm, or the back of the neck. The small touches Sam could handle, but most of the time, when not in public, Dean was no holds barred, wrestling the angel on to his lap, his arms locked firmly around his waist, their mouths always within kissing distance. It was sweet. For about a week. Then Sam couldn't handle it and began to get a separate motel room, partly for his own sanity, but also because the couple had awkwardly intimate moments that Sam didn't want to intrude on.
Such as the two of them slow dancing in Bobby's kitchen to rock ballads. Sam quickly snapped a picture of the pair on his phone, smiling once more before going off to find his own source of entertainment.