How dare he come back? After everything. After he left when Dean had told him he needed him. Twice. He certainly doesn't need Dean, he'd made that perfectly clear. So why should he be welcoming and friendly and act like everything is fan-fucking-tastic? Not this time. He's left too many damn times. It'll just make it hurt all the more when he inevitably leaves. Again.
However, Dean couldn't help but admit the immense relief he felt mixed with all of the other raging emotions running through his muddled mind when he saw that familiar figure lying in the middle of the road. And when he heard the voice he had grown so accustomed to hearing over the years say "A little help here." he almost smiled. Almost. But by this time his heart was so far up his throat he couldn't do anything except take a deep gulp and an even deeper breath. It wasn't until Sam sprang into action, running over to the figure lying on the ground, that Dean finally snapped out of his frozen stupor. And even when Cas had muttered "Hello, Dean." all he could find it in himself to do was nod with a straight face and get behind the wheel.
He spent that entire night awake, pacing back and forth, checking in on Cas every now and then. Because even though he was angry as hell, that didn't mean he wasn't worried. And he hated himself for it. He was supposed to be mad. He was supposed to be hurt and angry and ignore him and not give a rat's ass that he was hurt. Serves him right for not trusting him- for running away again. No. How could he ever think that Cas deserved to be hurt?
After they found that film, Dean knew it was Cas' first time ever viewing a movie, so of course they needed popcorn. And even though he wasn't hungry, and Sam was feeling nauseous, that still didn't stop him from popping the popcorn over the stove and plopping it down in front of Cas without saying a word. Even when he muttered a quiet "Thank you," and took a piece. He'd be lying if he said he didn't quickly peek at his face to gage his reaction- to see if he liked it. When Cas finished his first piece and grabbed a handful, Dean looked away, satisfied.
A part of him wished he hadn't told Cas that they didn't need him to come with them. Because as hard as he tried to deny it, now that he was back he wanted to spend as much time around Cas as possible. Even though he wasn't currently speaking to him, his presence held a certain comfort to it. A comfort Dean hadn't felt in a long time. But he knew that Cas needed to get better. He was still hurting and he wasn't healing as quickly as he should've. That and he was secretly afraid that if he spent too much time around him right now he would forgive him. And he wasn't ready to do that. Not yet.
When Sam told Dean he couldn't find Cas, he felt his heart drop all the way down to his feet. He knew that he should've expected it. But this time, as foolish as it was, he had really thought that Cas would stay. That he'd finally confess to needing Dean just as much as he needed him. That he'd do whatever it took to make Dean talk to him again, and to be honest, it wouldn't have taken much. Cas had always been the exception. Always his exception. He allowed himself one inner, pathetic sounding "Son of a bitch," into the depths of his mind before he regained his composure and casually told Sam that it "sounded like him." That part was true, even though every part of him wished it was a fallacy. Because those two times he had told Cas he needed him, he'd meant it. He'd opened himself up more than he had in a very long time, maybe ever. And he'd been rejected. And Dean hated himself all over again, because although Cas didn't need him, he sure as hell needed Cas.