Monsters, Villains –whatever you want to call them– they know. Exposition by a supernatural creature is par for the course on SPN.
But then the supernatural creature will take it a step further, to try and throw a wrench into the works, in order to get one or both of the lovers to comply, to bend to their will.
Now, we already knew that Cas loves Dean. And what EmptyCas said to Cas reminded me an awful lot of Naomi when she was trying to manipulate Dean.
They are trying to make them feel like their love is unrequited. This is the, “I know what you fear,” bit. As if they are saying, “I know you HOPE but… no.”
Octoswan has a great meta on classic romance tropes used between Dean and Cas. Check it out here because it is amazing.
Tropes used in 8x19 and 13x04: