What Dean doesn’t understand is that the reason he kept Cas’ trench coat after he walked into that lake was because some of his celestial essence remained, like an angelic stain, like a feather left by a bird.

What Dean doesn’t understand is that he’s become dependent on that source of security, that his body craves it.  His body relaxes when Cas is around, possibly because Cas would watch him while he slept, possibly because Cas had gone back in time and introduced himself to Dean over and over again - having been accidentally found by the toddling future hunter a few too many times, possibly because it feels like him.

What Dean doesn’t understand is that carrying that coat, wrapping himself in it as he slept, and holding it while he cried also imbued it with the essence of his humanity, his soul.

That trench coat is a beacon to all of heaven and all of hell and all of the privy supernatural that Dean Winchester and Castiel are connected, marked, taken.

It pisses off all of them.  Some are jealous; some find it despicable.  Some find it intriguing.

Worst of all, Cas knows that Dean doesn’t understand.