When Castiel fell and became human, he got needs. Like the needs normal people have. He needed to eat, sleep, take showers, and of course use the toilet. Dean taught these things to Cas, with a little help of Sam. The only thing Cas hadn't learned was how to brush your teeth. Dean found this especially annoying, because Cas was always intruding in his personal space, so he was the one who had to smell the breath of the fallen angel.
"Cas, seriously, when was the last time you brushed your teeth?" Cas tilts his head, eyes full of confusion.
"Okay, so never…" Dean lets out a sigh and leads the confused Castiel to the bathroom.
"Come here, stand next to me. Luckily I've got this extra brush…" Dean mumbles as Castiel continues staring at him frowning.
"Okay, take this toothbrush, here. And then, put some toothpaste on the brush, like this…" Dean shows Castiel every step with his own toothbrush. Cas copies his every move.
"And then put the brush in your mouth, and then start to brush your teeth. Look! It's easy!" Dean chuckles the brush in his mouth.
"I don't understand… Why I have to put a brush into my mouth?" Dean spits and sighs.
"Dude, oral hygiene! And believe me, chicks likes a fresh breath," Dean grins. Cas just nods, and puts the brush into his mouth.
"Then you just… Have to move it, so that you brush your teeth, you know," Dean tries to explain. When Cas looks more or less lost with the toothbrush in his mouth, Dean decides to help. Cas is his friend, and he really need to learn the human customs.
"Let me help you. Like this…" Dean grabs the brush and starts to move it brushing Cas' teeth. For some reason Cas seems to enjoy Dean brushing his teeth and the ex-angel lets out a quite moan. Dean just stares at his friend confused, but keeps going.
"You know, if you never learn how to brush your teeth, how do you think you're ever be able to learn to give a blow job?" Dean teases a grin on his face.
"I have to learn that, too? Does humans do that a lot?"
"What, blow each other?"
"Yes. You once told me to blow you. If I learn how to brush my teeth, will I be abel to blow you?" Castiel sounds so serious that Dean without thinking just nods.
Suddenly Cas is much more eager to learn how to brush his teeth.