Suffice it so say, Castiel isn't well versed in human doings. Dean always knew Cas didn't have the hang of some of the finer points of mortal life, but he had sort of assumed that Castiel would know how to shave. You would think purgatory would have prepped him for that realization.
After a strangely uneventful month of Cas living with them in the bunker, Dean finally takes notice of Castiel's face. It's more than a little hairy. Dean automatically schedules a time in his head to drag Cas to the bathroom and teach him how to shave. They can't be seen living with some caveman. Call Dean what you like, but he at least he maintained his body hair.
To be honest, Dean is surprised that Cas got along this far without someone noticing his surplus of whiskers. He would have thought for sure that Sam would have taken it upon himself to guide Cas through shaving. Then again, who knew the last time that boy had a haircut, much less shaved his face.
Dean decides to corner Cas later on that night. They leave for a hunt tomorrow and need to be taken seriously as agents; not some hooligans who don't shave their damn face. Sure they occasionally used semi-ridiculous names as a cover, but only well versed agents tended to pick up on that. No self-respecting FBI agent would be caught dead with a beard as fluffy as Cas'.
When the hour is finally upon them, Dean abruptly approaches Castiel and yanks him out of the chair he's been reading in for the past hour. That's something Castiel has done a lot of since moving in. Half of his free time is spent reading whatever collection of books he's stumbled upon that day in the enormous Men of Letters library. There are certainly enough books in there to keep Cas busy well into his old age. If Winchesters (honorary or not) got to that age.
Reading or not, the time has come for Castiel to finally clean up his cheeks, so Dean takes the startled and squinty eyed Cas to the nearest bathroom and announces his intentions to teach him to shave. As expected, Castiel doesn't exactly get the hang of it immediately. It's a fight to get him to do it all, but once he sees that Dean mean business, he backs down and picks up a razor.
Shaving cream alone practically breaks Dean. With this idiot smearing it everywhere, Dean almost lets him grow it out, but gathering up his resolve, he takes the baby in a trench coat and guides him through it. It's tentative at first, with Dean awkwardly positioned behind Cas to help him. Blood loss is minimal, but that doesn't make the experience any less aggravating.
In the end it takes Castiel somewhere over two hours to shave, but Dean can't say he didn't enjoy it. Between Cas getting shaving cream up his nose and huffing angrily when he couldn't remove the hairs, Dean checks the experience as an enjoyable one. He doesn't know about Cas, but now that his face is baby smooth, at least Dean can stand to take him out in public.
He thinks that Castiel must somewhat miss his scruff because he catches him several times almost petting his face with an almost forlorn expression. On the other hand, Dean doesn't give a damn because if Cas is going to live under his roof then his face would be kept clean. Over time Cas would get better at shaving and they could all live happily ever after. Yeah right.