You have to explain the concept of nicknames to him.

Castiel and you were taking a walk through a park together. It was the middle of summer and happy couples and families could be seen everywhere.

You were holding hands and you smiled up at him. "When was the last time you took a walk like this? I mean, not to get anywhere, just for the fun of it."

"I'm not sure I've ever done that, to be honest", he said and you leaned your head onto his shoulder.

"So are you saying that your life so far only consisted of work, fighting and slaying?"

"Basically, yes", he said and turned his head to kiss you on the forehead, which felt wonderful.

"Aww... you never got to have any fun. Poor baby", you cooed, because you were in a playful mood. "Poor Cassie."

Castiel stopped at once and you took your head off his shoulder. "What?"

"That's not my name", he said and wrinked his forehead. He looked so irritated, you couldn't help but smile.

"I know. I just wanted to call you that, because it's cute."

He cocked his head, piercing you with those deep blue eyes of his (or Jimmy, to be exact).

"Why would you call me by something that's not my name?"

You giggled, then took his hand and pulled him towards a bench to sit down. "C'mon, people are staring."

Other couples or parents with their kids had in fact stopped to look at you when Cas had stopped, walking abruptly, causing a sort of traffic jam in the crowded park.

It might also be because of the fact that he was wearing a trench coat on a hot summer day, you mused.

Lucky for you, an older couple got up right when you approached the bench. They smiled at you and you returned the smile as you sat down and pulled on your boyfriend's wrist, until he sat down next to you.

"My name is Castiel", he said randomly.

"Um... I know that?"

"I was just making sure you do, because you called me something else before."

You laughed. "It's called a nickname! Have you seriously never heard of that?"

"People have names", Castiel said stubbornly, "to be addressed by. What would be the point of calling them something else? It's confusing."

"Look", you said, while playing with his hair, which was warm from the sunlight. "Dean always calls you 'Cas', right?"
"Yes. And so does Sam occasionally."

"See? That's a nickname right there."

"I always suspected that they simply derived it from my full name, as some sort of abbreviation."

"Yeah, right", you encouraged him. "That's basically it."

"In battle", Cas said, while putting the strands of hair that you had fiddled with back into place, "it can be useful to call out a short name that consists of only one syllable. The names 'Sam' and 'Dean' are already short, you see?"

"I don't think that's the only reason why they call you 'Cas'", you smiled. "You know how Dean often calls Sam 'Sammy'? That's not shorter or more convenient! It's just meant to show his affection."

Cas frowned. "How does the addition of one syllable to a name relate to affection?"

"It just does", you said with a shrug. "So when Dean calls you 'Cas', it means that he likes you, that you're his friend, you know? And when I called you 'Cassie', it was to show how cute you are to me! And when I call you 'babe' or something like that, it means I love you."

"Babe", Cas repeated tonelessly, staring at a point in the distance. "Dean used that word once to describe a girl we met at a bar."
"Oh really?", you asked innocently.

"Yes. He pointed at her while she was dancing and said: 'See that one, Cas? That one's a babe.'"

You raised your eyebrows. "While she was dancing, huh? What kind of anbar was it?"

"Dean called it a special place, where he would like to take me to help me 'losen up'."

You snorted. "Alright, I don't need to hear the details." A little suspiciously, you added: "Was that before my time?"
"No. I'm certain you were already born then."

"I meant before you and I started dating!"

"Oh. Then the answer is Yes."

You smiled and gave Cas a kiss on the cheek. He hadn't shaved in a few days, but you didn't mind.

"I love you, honey", you whispered. He turned his head to face you.

"Honey."
"Yes. That's also pretty common as a nickname", you defended your choice of words.

"But it's something to eat. And it's sticky."

"It's also really sweet!", you said defensively.

"Which is why it can cause diabetes", Cas said, once again brutally honest and apparently not even aware of it.

"Okay. So 'babe' and 'honey' are off the list, huh? How about 'love'?"
"Confusing. Love is the feeling you have for me, not my name."

You pouted. "There's not much left. Only really ridiculous things like 'cupcake' or something."
"So humans prefer to call their loved ones after food that contains tons of sugar and thus can lead to cavity, obesity and more serious health issues? That's not very romantic."

You let out a sigh, but still smiled. "There's still 'sweetie' left. That's not a food."

"Or you could just call me Castiel", he suggested and made you laugh.

"I'll call you Cas like Sam and Dean, alright?"
Castiel nodded. "I'm already used to that."

"And every once in a while, I'm allowed to give you a cute boyfriend nickname, right?"

"I suppose", he replied and smiled at you.

You kissed and he took your breath away. His hands slid up your arm, then you felt his hand in your hair. After a while, you pushed him away gently and whispered: "Not appropriate. We're in public."

"Then let's go home", he said with a grin and you agreed.

"You know", you said, as you began to walk out of the park again, picking up on the previous conversation. "You could also try to give me a girlfriend nickname now and then."

"Alright, I'll try", he promised and you felt really happy...

...until the next morning, when he woke you up with the words:

"Good morning, artificial sweetener!"

You blinked, still too tired to fully grasp what had just happened. "Um... what did you just call me?"

He beamed down at you and ran his fingers through your hair lovingly. "It's sweet and it doesn't cause diabetes. Would you like some breakfast, artificial sweetener?"