AN: So I kinda just had an idea then all this angst happened.
It was when Dean reached up to close the trunk of the Impala one day that Cas noticed something terribly wrong. The skin visible on the hunter's hip bone was decorated in fine scratches of angry red, white, and pink that receded down his side. Ever since Castiel met Dean, he knew the hunter had gone through what no righteous man—especially the Righteous Man—should have to endure. Dean was apparently struggling much more than Castiel had initially believed.
The first chance Cas had to speak to Dean alone, he took it. The Winchester's were at the bunker and Sam was conducting research on a wraith hunt he found while Dean confined himself to his room. After a knock on the door to announce his presence, Cas received a "Just a minute," from Dean. When Dean opened his door, Castiel stormed inside the hunter's room without invitation. When the door was closed, he glared daggers at Dean.
"Dean Winchester, how dare you put yourself in such danger!" He yelled. At Dean's confused look, Cas stepped forward and wrenched the man's shirt up to reveal the scars and a few new bleeding cuts. The color seemed to drain from Dean's face at the realization that his secret was out.
"You are a good person, Dean. Why would you harm yourself in such a manner?" Dean was avoiding Castiel's fiery cerulean gaze like the plague, but still gave a weak response.
"You wouldn't understand, Cas,"
"Oh really, Dean? Try me. Because I don't know if you've noticed but I just lost my home and I'm an entirely new and frustrating species than I was two years ago. Not to mention that I was the one who destroyed Heaven in the first place!" Dean, rightfully so, looked a bit shameful at that and still not meeting Cas' eyes, spoke bitterly.
"Well, I screwed up again as always: I let Sam be possessed so that the angel inside him killed Kevin and Sam is angry and there's angel armies everywhere and everything seems to all come back to the Righteous Man who broke the first seal. So yes, I have been hurting myself, but it's nothing new. I've been doing it for as long as I can remember."
Castiel's heart wrenched with every word that left his friend's mouth. Why would Dean to this to himself?
"For as long as I have been on Earth, things have confused me, but this more than anything else I don't understand. I don't understand why you can't see the good in yourself. I don't understand why you think everything is your fault. But most of all, I don't understand how you could do this when I am right here. Because for as much as you have been through, I have done and seen even worse."
Throughout Cas' rant Dean stayed silent and grew a look of guilt. "No Cas, see this is one of the reasons! You put too much faith in me. You have all these great expectations and I can't meet a single one. So I'm sorry I'm not good enough, but I never was."
Cas shook his head in disbelief, "How can you think so poorly of yourself?! I do not regret even for a moment losing everything for you because despite what you believe, you are worth it! The only thing I regret is betraying you and losing your trust. So please, Dean, let me help you. Let me teach you the truth about yourself that for some reason you can't see. Let me be here for you, and please don't ever harm yourself again."
At this point, tear's were shining in Dean's eyes so Cas followed his human instincts and wrapped the hunter in his arms and whispered into his ear, "Dean, I care very much about you and it pains me to see you hurt physically or otherwise." Dean just nodded, accepting the hug and the words along with it.
What Cas confessed earlier made Dean want to reassure his friend.
"I'll try, Cas. I can't promise anything, but I will try. I don't want to cause you any sort of trouble and I'm sorry that I did but I will do my best."
Cas' simple reply meant the world to Dean, "Thank you, Dean. That is all I can ever ask."