When Castiel pulled out the chair for Dean the sixth time in the last three days, that's when Dean knew something was up.

When he held open the doors for him everywhere they went, that's when Dean got a little concerned.

When he opened the Impala door for Dean and stood there waiting for Dean to slide into the drivers seat so he could shut it, Dean drew the line.

"Look. Dude. You can tell me if something's going on." Could angels die of cancer? Mourn the lack of chivalry in the world?

Castiel frowned at him, "Dean now isn't the best time…"

Dean crossed his arms, "I call bullshit. Cas what's going on."

Cas played with the hem of his coat and his lips tightened. "please Dean… Later."

And, since Dean could only say no to puppy eyes for so long, he gave in.

And got the fucking surprise of his life when he opened the door to the room Castiel had got them and found it was the honeymoon suite.

Rose petals, chilled booze, heart shaped bed, the whole nine yards. Dean understood why Cas gave Sam a separate room better now.
"Dude what the f-"
"Dean we have known each other for the better part of four years now and I have something I need to tell you." the beginning of the speech was so pre written and rehearsed it actually caused Dean physical pain. "You are one of the few people I find I enjoy being around. You constantly amaze and surprise me. Your soul lights up the darknes-"

"woah woah woah. Cas." he held up his hands, "Are you trying to confess your love for me right now?"

Cas' mouth fell a little open and he rubbed the back of his neck, "I… Yes, Dean."

"And that was what the whole holding doors and pulling out chairs and helping me with my coat thing was about?"

"…yes? Is that not customary? I was under the impressi-"

"Who gave you this fucking advice man." Dean demanded.

"Sam." he sounded relieved to give Dean an answer he knew for sure.

"That son of a bitch." Dean said, as he turned on his heel and swept down to 4b.

"Dean where are you going?" Castiels voice held a hint of panic.

"To go deal with that jackass!" he said turning to find himself nose to nose with Castiel. "He gave you some freaking horrible advice man. That is not how you tell someone you love them, okay?"

Castiel's brow furrowed, "then how-?"

Dean kissed him, quick and hot, a hand careful on his neck.

"That's how dumbass. Now I have to go beat my brother senseless."

Cas looked sort of dopey with a smile that big on his face, but the more Dean looked at it the more he thought maybe it suited him.

Later Sam told him he gave Cas horrible advice because he knew he didn't have to win Dean over, he'd had him from 'Castiel, angel of the lord'.
Dean gave him a black eye and a grudging hug. And another smack for being so fucking sappy.