Cas was trapped. And alone. He was alone. In the dark. With literally nothing else than him. Not even his shadow. For the very first time in his life, he was completely alone and yet, he felt more threatened than never.
Being alone meant no Dean. No Sam. No nothing. But especially no Dean. When Dean wasn't there, Cas'd just do stuff that'd remind him of Dean. Driving fast while listening to his special tape very loudly, eating pie or burgers, drinking beer, watching Netflix or, even better, calling Dean and talking about nothing. Nothing was cool.
Now nothing was everything he had around.
He had already died, it's not new. But this time is different. He died watching Dean being happy that he was alive a few second before getting killed by Lucifer. And now everything is empty. Nothing.
He can't seem to escape. Everything is made of nothing. How can we escape nothing ?
He wants to go back and hug Dean so hard. He wants to kiss Dean so hard. He wants to do everything he has thought about doing with Dean. Everything.
But he's trapped and can't seem to escape.