"Cas! Castiel!" cried Sam, trying to be heard over the crackle of static and echoing boom of Castiel's voice. "Cas, I'm sorry! I know!"
The darkness in the room lifted, and Castiel's eyes returned to normal. He appeared again as merely a man, disheveled and sad. Tears welled in his eyes, and he tried to sniff them back in. "What, what is happening to me, Sam?"
Sam's long legs brought him to Castiel quickly, and he embraced the angel, pulling him into his chest. "Cas, you are feeling sad."
Castiel said nothing for a moment, just sniffed, then replied in a low voice, "I think this is different, Sam. I felt sad when my brothers and sisters died. I felt sad when humans died because of my failures. This… this hurts. This is not a feeling angels have." Castiel gave Sam's arm a squeeze and then stepped back from the younger Winchester.
"No, Cas, I don't think it is. It's a feeling humans have. It's… it's like loneliness and helplessness and adoration all mixed together." Sam swallowed hard. "You love him, don't you?"
"No," Castiel shook his head. "I know love. I love our Father. All angels do."
Sam made eye contact with the angel, "Cas, aren't you born with that love? It's automatic. You didn't choose to love God." Sam sighed, "you choose to love my brother, and that's why you feel so torn up inside. So helpless. God's love is a constant. Dean's love is… hidden."
Castiel didn't say anything for a while. Sam let him. Sam's mind was reeling with the possibilities and implications. He'd known Dean's feelings for Cas for years, but what could he do? Dean would never talk about it. And even if he could talk about it, what could he do about it? Start a relationship with an alien from another dimension who also, by the way, inhabited a male vessel?
Castiel finally broke in, "Sam, my feelings for your brother are... impossible to rectify. The best I can do is continue to care for him and guard him," he said as he composed himself. "We need a plan. He looked directly into Sam's eyes, "I need his permission to enter his dreams. I believe I can help him recognize these manifestations to be emotions that he can process and let go."
"What would that look like, exactly?" asked Sam.
"He'd need to give me permission to enter his dreams", Castiel said in a low, quiet voice. "He'd then need to go to sleep. Before, when I would force my way in, it was like moving through a viscous medium, and I couldn't take my full form. I was a shade, and it took all my effort to corporealize long enough to take him out of it." Castiel ran his hand up through his dark hair, and looked sadly into Sam's brown eyes. "He fought me, Sam. Parts of him didn't want me there. He wanted to be where he was, do what he did. He is coming undone."
"He's writing a narrative where he is the villain," Sam interjected. "And you're worried he's going to start believing it."
"Yes, that is my fear," Castiel replied sadly. "If I can enter his dream with his permission, he'll see me as I am, in my true form."
"So you'd look like you, not a shade?"
"No, Sam. He would see me. My wings, my real voice. I would appear softened, muted by his subconscious, but he would see me for what I am." Castiel's voice sounded tense with anxiety as he continued, "I fear it will overwhelm him. I fear he will be... afraid of me."
"But," Sam added hopefully, "it's the only way to get him to talk through this."
"Yes," Castiel replied quietly. "If I can show him that he's not the man he thinks he is, but instead the man-"
"That you love?" Sam finished the sentence. No one said anything for a moment.
"Yes, Sam. That."
Sam turned and walked toward the hallway, "lemme throw on some actual pants. I have some ideas about where he might go."