It must be his vessel. That's the only explanation he can find for it; it's the only reason that he stutters and stumbles every time Sam enters the room. He knows the basics of the body, but there is still a lifetime of experience to catch up on. That's what makes him sure that his vessel is most certainly malfunctioning, makes him sure it's not just his mind.
Angels can't feel emotions, right?
But he knows, deep down, that that isn't true. He knows the stories, the facts gone myths, the legends gone folk tales, of angels banished for their willingness to feel, to love.
And he knows it's wrong.
That's why he refuses to admit, even to himself, that there is a tiny spark, deep down.
Castiel sighs and continues fighting with his comrades. They can't let the others win; they can't let anyone other than God take back charge of Heaven. And it's not like anyone actually knows where the Hell he is anyway.

Almost.

The angel blade misses him by no more than an inch and it pulls him right from his train of thought.

Focus.

He can't lose here; the boys need his help, just as he needs theirs. He has to get back to them one more time at least. He has to find out what's wrong with his vessel. He knows he'll ask Dean, although there's no guarantee that the man won't just laugh at and tease him. The thought makes him smile, until he remembers that that is forbidden, and so it turns into a grimace. Emotions can wait till later.

It's been a long day, and now he's standing here, the two brothers staring at him expectantly. He listens as Dean explains that there's something wrong with Sam, and he nods before going through all the possibilities in his head. Grimacing as it comes to him, he hands Sam a belt, telling him to bite down on it because this might be painful, and it hurts inside his chest for some reason, and he's sorry.
The howls of pain are unbearable.
His soul nearly shatters into fragments as he searches for Sam's own, and continues to find nothing. Pulling his hand out of the man's chest, the howls dying down into silence, he explains that he found nothing and that's exactly the problem, he's sorry but Sam's soul is long gone.
The looks he gets from the brothers leave him with a feeling of despair, and he sighs. It's been a long day.

He hears his name, and it's not the usual voice he responds to that calls him, but a younger, less rough one.
Castiel.
He hesitates. Should he go to him, and face the problem he's been trying to avoid? He's been meaning to go ask Dean about it, but then again, hasn't he been avoiding that too? Sighing, he extends his wings and soars. He's standing in front of Sam within seconds, and he watches as the man jumps in fright at his sudden appearance. He smiles, slightly.
"Hey."
It's so good to see him whole again, to hear his voice, and he can't help himself as he steps forwards, arms outstretched to catch him in an embrace, but he only gets so far before Sam sits down hurriedly, refusing to meet his gaze. Castiel lowers his arms awkwardly and backs away, a sinking feeling in his chest. Sam asks him what happened over the past year, and although Castiel is reluctant to tell him, he does so, because it's the least he can do.

Dean confronts him later without his brother's knowledge, and asks him why he has been acting all weird around Sammy recently because it's really freaky, and all that he can say is that he doesn't really know but could Dean help him because vessels are not supposed to malfunction around certain living beings. Dean stares at him and then bursts out laughing. Castiel is tilts his head in confusion because usually humans are meant to worry when one of them becomes ill, are they not?
But he's not one of them.
Dean's laughter subsides and he looks Castiel in the eyes.
"You're not in love with my brother, are you?"
Of course, the angel vehemently denies it, even as he himself finally knows it to be true. He flies far, far away, away from everyone and everything that reminds him of Sam, Sam and his smile, his laugh, his eyes...

The angels find out soon enough, and Balthazar comes to him, offering a choice. The angels will kill Castiel, and he knows this. He can either stay here and wait for death, or tear out the Grace within and fall to Earth to live a human life, hidden from all angelkind. He knows he can't go back there, freshly human, and put the Winchesters in danger, but he doesn't want to die. He tells Balthazar that he will stay here and wait for what will happen, but somehow they both know he's lying.

It's midnight when he slips past the masses of warriors battling and makes his way out of Heaven. He looks back one last time at what was his home for so many millennia and sighs, his heart sinking in his vessel's oh-so-fragile chest. He spreads his wings and soars.
Sam.
He needs to see him, needs to tell him how he feels because it's never been as crystal clear before. The angel lands where the brothers were last, but there is no sign of them, not anymore. He sinks to the ground, his heart rising in his throat as he sees two of his fellow angels descending from the skies above. He surrenders, knowing now that he'll never see Sam again, because he can't track him, and he's going to die without being able to tell the only man he's ever loved that he cares about him. He stands. The angels beat him and leave him in a circle of holy fire, blood flowing down his face, and he blacks out.

The purr of an engine seeps into his dreamless sleep, and he stirs without waking.
"Cas!"
It's that voice, that panicked tone, that makes him force open his eyes and gaze up into the two emeralds in front of him.
"Sam..." he groans weakly, as the man's eyes well up, tears threatening to spill. "Sam, it's alright..."
Sam shakes his head and tilts Castiel's face so he's looking at him. He tells him something about staying awake and alive but the angel isn't listening; he's too busy studying his eyes. They're bright green with flecks of brown and blue and gold, and they are the most beautiful things Castiel has ever seen. They're even more beautiful than Sam's soul, and despite it feeling like it had been skinned alive it was still wondrous to behold; perhaps that was one of the reasons it was so amazing anyway, for what could withstand such tortures and still remain good and pure? Castiel knows that one can see the soul through the eyes, and if he looks a little deeper into Sam's, he can see the sorrow and guilt that haunts him and always will, but even deeper than that is the wisdom of a man who has walked the Earth for centuries and learned all his lessons in the hardest possible ways.
"...Cas? Are you listening? Cas!"
Sam's words have twisted into sobs now, and Castiel knows it's because of him, but not how to deal with it, so he does the only thing he knows how; he pulls Sam's head down gently, their faces millimeters apart, giving him plenty of time to back out, but he doesn't, and it's better than the angel could ever have imagined, and their lips press together, speaking thousands of words that neither would know how to say. Castiel smiles, because he's said what he needed to for so long, and he lays his head down on the ground again. He doesn't know when the holy fire got extinguished, but it's probably something to do with the water (rain?) all over the tarmacked yard. He looks up at Sam and smiles again as he fades away.
I love you.

Death stares at the angel, who is now looking over his vessel's lifeless form, and rests a hand on his shoulder.
"I will give you one more chance, Castiel. Your time is not yet here. I can erase these events from your memories, and you will think you just spent the night lazing in the motel."
Castiel nods, a lump in his throat. "Thank you."
Death only gives him a slight smile, touching two digits to his forehead.

"Hey Cas."
"Hello Sam."
The angel feels strange today. He had a dream about his own death and no doubt that is what's making him feel this way. Perhaps he should tell Sam that he loves him, that he is in love with him, because he feels like it's about time Sam knew the truth. He doesn't though. The younger Winchester obviously notices his pondering, because he asks him what the matter is, but all Castiel does is smile and shake his head.
"It's nothing."
It isn't nothing, but he doesn't want to say it now. He's got an idea for how to do it and it can't happen today.

'I'll tell you tomorrow,' he thinks. 'Even if they kill me, I'll tell you.'