Cas didn't understand emotions, or how humans dealt with them.
They didn't mean anything. He was sure of that. Feelings were just that... something you feel. They don't change you.
Cas was still the same angel he'd been 1,000 years ago. He still obeyed and loved god. He still would do anything for his father. He was still the same Castiel.
His father, God, hated the nickname Cas. He'd told Castiel that the first time Dean had said it.
"Castiel, you're an angel." He had told him, "Don't ruin yourself with human nicknames. You aren't a human, Castiel. You're an angel. Don't forget that."
Don't forget was something Castiel heard a lot. He couldn't forget that he was an angel. He couldn't forget he wasn't like Dean. He couldn't forget than angels don't fall in love with humans. That was the problem, because Cas did forget.
He forgot that he was an angel every time he was near Dean. Because Castiel ruined everything. He always did. Dean ruined things sometimes, too, but Dean regretted those things. He felt like it was his fault. He knew how terrible they were. Cas didn't feel like that.
Whenever Cas was around Dean, Cas felt like Dean was the angel (but he'd never tell.)
"You can't forget." Cas whispered as he sat down, his back pressed against the cheap motel wall. The uncomfortable textured wall dug into his back, and a slow rhythmic thumping alerted him that yes, the people in the room next to him were indeed having sex.
His small cell phone, which he still wasn't quite used to yet, rang from the bedside table next to him. Cas jumped, stared at it, then picked up slowly.
"Who is this?" he asked the caller, not even bothering to check the number on the small screen.
"Cas? Who do you think?" Dean's slightly angry voice asked.
"I wasn't sure, that's why I asked you." Castiel replied, feeling guilty.
Dean sighed on the other end, then paused. "Sorry Cas, I shouldn't have yelled. Anyway, I was just asking if you could hold down the fort for the night. I met this smoking hot girl, and uh, we're going to have a very busy night, if you know what I mean."
Yes, Castiel was fully aware of what Dean ment. He didn't like it at all, but he didn't say that. He just didn't say anything.
"Yes... I can watch the hotel room." Cas said finally.
"Great. I owe you one." Dean said quickly.
"Goodbye." Cas said quietly, already knowing Dean had hung up again.
Cas tightened his tie and looked down at his trench coat. The one he'd worn the day he met Dean.
Castiel didn't understand emotions, much less feel them. Because he was an angel, not a human. Angels don't feel emotions.
So why was he so upset every time Dean took a girl home?
Maybe some angels did feel emotions, Castiel decided.
He also decided he wished they didn't.