Behavioral Analysis
Castiel didn't always understand Humans. In fact, more often than not, he was quite confused by them. They had a wide range of complicated, often illogically overlapping, emotions. As an Angels, these emotions were hard to decipher. It didn't help matter that humans had a tendency to say and do the opposite of what they actually felt.
"When humans want something, really bad, they lie," Dean had once said. This practice thoroughly confused Castiel, but perhaps at least knowing that it was common would aid him in deciphering the actions of Humans.
Castiel was closer to Dean than he was to any other Human. He knew Dean quite well, but still he did not understand him. He did not understand why Dean was behaving so strangely, as of late. There was no obvious reason for his actions. As far as he could remember, he had done nothing to put himself on Dean's "shit list" as the hunters called it. But Dean had never outwardly said anything to indicate why he was acting so strangely, so Castiel brushed it aside. There were more important matters at hand.
Except, Sam had noticed too, and he wasn't going to let it slide so easily.
"Man, why are you acting so weird?" Sam asked. Castiel was gone at that point, or so the Winchesters believed. He watched them invisibly from the doorway. Perhaps he had done something wrong, surely Dean would tell his brother, and then Cas could fix it.
"What are you talking about?" Dean asked.
"You're really weird about this whole Balthazar thing," Sam said.
Castiel had almost missed the connection between Dean's odd behavior and their recent encounter with the other Angel. Almost. But logically, there was no reason for Dean to be acting so strangely. Sure, Balthazar didn't share the same values as Dean (okay, so he'd tried to kill them on at least one occasion), but the same was true for most of the Angels, and Dean had always been openly hostile toward them. Not passive aggressive toward Castiel, as though it was his fault.
"I don't know," Dean shrugged. "I guess there is just something about him…"
"Is that some sort of euphemism for you want him in your pants?" Sam asked, grinning mischievously.
"Dude," Dean groaned. "Eww."
"Well, you never know!" Sam shot back defensively. "It's not like he'd be the first angel you wanted to bone."
"And what the hell is that supposed to mean?" Dean asked, raising his voice slightly.
"You know. Anna."
"Right," Dean nodded distractedly. "Anna."
"And," he added slowly. "Cas."
"What the hell are you talking about?" Dean asked, not moving his focus from the bag he was packing. This was not the outrage that should have come, but avoidance, which Sam seemed to be expecting.
Castiel, however, was even more confused. What exactly was Sam trying to imply?
"Oh, come on, man," Sam said. "You clearly… oh. Oh."
"What oh?" Dean asked, not making the connection the Sam seemed to have found with ease. Castiel couldn't figure it out either.
"That's why you're acting weird."
"Dude, I don't know what you're talking about."
Sam laughed. He actually laughed. "You're jealous!" he exclaimed. "You're jealous of Balthazar. Of Cas and Balthazar."
"What the hell does that even mean?" Dean asked, though it was obvious he wasn't nearly as confused as he pretended to be. Obvious to Cas anyway, and probably obvious to Sam.
"Cas and Balthazar are pretty good friends. Have been for a while. You're jealous 'cause you think they've been giving each other a little action upstairs."
Castiel understands Human euphemisms. Not all of them, of course, but he understands this one. While Angels seldom participate in such activities as what Sam was implying, Balthazar had always been somewhat of a rebel in that department. But Castiel had never engaged in these activities, and he was confused as to why Dean would be jealous of that assumption.
Unless…
"Dude, are you ever gonna just grow a pair an tell him how you feel?" Sam asked.
Oh.
Dean let out a low growl and threw his duffle at Sam, who just barely caught it.
"Go pack the car," he snarled. He didn't deny Sam's accusations.
Oh.