Cas was walking through the bunker, headed to the kitchen, when he passed Dean's door and heard sounds coming from behind it. He backtracked and listened at the door, and heard a beat he assumed was from music coming through it, along with what sounded like Dean, singing.
Curious, he opened the door a little and looked in.
Dean must have been in the process of getting dressed, because his hair was wet from a shower and he was only wearing jeans.
He was facing away from Cas, doing what Sam had described to him as "jamming".
So raise your glass if you are wrong
In all the right ways
All my underdogs
We will never be never be
Anything but loud
And nitty gritty
Dirty little freaks
Dean turned around and spotted him.
"Hey Cas! Come on, jam with me." He grinned, and while Cas was pleased his human was happy, he didn't know how to "jam", and was very confused.
Dean finished his "jamming" a moment later, apparently the song had been just about completed anyway, and watched Castiel.
"So what's up?"
"I heard noise and thought I should investigate." He cocked his head to the side. "You are a very good singer Dean."
"Thanks Cas." He thought he saw his hunter's cheeks flush a little, but it may have been a trick of the light.
"I do not understand the song… It sounded triumphant, but the singer called herself and her friends dirty little freaks."
"Cas look at me and Sam. We're freaks. You're a freak too Cas, hell, you ran away from home and you're constantly breaking the rules." Cas started to feel like he should be offended, but Dean continued. "You just gotta embrace it sometimes. Sure, I'd like to be normal, me and Sam have a normal life, but sometimes you've just gotta embrace the inner freak."
Cas nodded slowly, kind of understanding, and Dean turned to put a shirt on.
Cas didn't understand the tiny twinge of disappointment he felt as the shirt covered his hunter's chest.
"C'mon Cas, let's go grab some chow." He clapped Cas on the back and left, and Cas followed him out.