Okay, so, I can totally see Cas as a tea drinker.
Just bear with me here...
Its more relaxed than coffee, and there's more types. Cas would be adventurous with his tastes and he'd probably have a new favourite every week.
He'd hardly ever be without a cup of tea, or whenever he was drinking it would be whatever the flavour of the week was.
And Dean would notice, of course, he spends so much time looking at Cas he'd notice. And he'd get a whiff of something fruity as Cas walked past and ask "hey, what're you drinking, Cas?"
And Cas would respond with, "my favourite tea, the berries one, Dean."
Dean would get confused, remembering the conversation last week, "wait, I thought your favourite was that whatsit lemon one?"
Cas would smile gently at him, "no, that was last week, this one's fruity," and then potter on his way, probably to help catalogue some more of the MOL files with Sam.
Dean would try to keep up and make sure to buy Cas' favourite, but 'cus he changed it every week, Dean would give up trying to buy the one Cas said is his favourite, and just get a different tea for him each week.
And whichever tea Dean picked out would be Cas' favourite.
Even if he bought his own because the flavours appealed to him.
The one Dean picked would be his favourite that week.