Cas doesn't understand nicknames or pet names or any sort of titles people ascribe to other people they are friends with. He always calls Dean as Dean but Dean calls him all sorts of names, buddy, pal, sunshine. To make Dean feel more secure in their friendship, Cas starts to call Dean by some endearments. "Love" is first one and Dean and Sam both spat their coffee out when Cas greeted Dean with his morning rumble of Morning Love, and it escalates from there, from Honeybee, to Sweetheart. All of it. It is a weird sort of flirtation, Dean blushes but never corrects Cas. It reminds Dean that he is worth the love, that Cas thought about him enough to give him a nickname.
