Okay but imagine this:
Dean is trying to flirt with Cas, trying to make him blush and using pick up lines, but Cas doesnt understand It so he tries to do It back and ends up saying something much more deeper than expected.
Cas: "People have told me before my vessel is asthetically pleasing, but why are you bringing It up now Dean?"
Dean: "Im trying to flirt with you, you know? Like when you like someone so you complement them hoping they like you too?"
Cas: "I understand."
Dean: ...
Cas: ...
Dean: "So.... Are you going to flirt back?"
Cas: "I dont know how to flirt Dean."
Dean: "Just say something nice about me, come on."
Cas: "What should i say?"
Dean: "Anything, tell me im beautiful or that you like my eyes."
Cas: ".... I.... I dont really know if your physical appearance is atractive, as im an angel and i dont see your body. But i see your soul.... And i do see how bright It glows even after all the bad things you've been throug.... Its the most pure and beautiful soul i have ever seen... As I see it... Its even more beautiful than the grace of a lot of Angels i have met. When im near you i can feel It, i can see It, It makes It dificult to see anything else.... So you could say your soul is atractive.... Or at least It atracts me."
Dean (not expecting that and with tears in his eyes): "Wow"
Cas: "Did I do It right?"
Dean (still not believing what he just Heard): "..... Um.... I.... Yeah.... Yeah you did great Cas."
Cas (smiling proud of his self)