The Doctor may have had a few nasty run-ins with angels in the past, but after Castiel's utter cluelessness while playing "FBI Agents" with Dean, I couldn't help but think that these two needed to be in some fanart together. Plus, the Doctor would totally be super nice to him, cause Ten is a nice guy (unlike Dean, who, hawtness when paired with Cas aside, is pretty much always set to 'slightly douchey' mode. No offense).
His psychic paper is psychic, bitches. So he can hold it any damn way he pleases! Unlike our poor baffled angel... and of course I adore K-9, so he's in there just because he's awesome like that.
The Doctor and K-9 are © BBC
Castiel is © Kripke