Dean was grateful to Cas for getting him out like he did. He had seen how difficult it was for the angel. Understood it a bit better now than he had before, after the two of them had spoken after Buffy had been taken back. It had been a thought in the back of his mind but it never fully hit Dean that the other angels weren't just douches that Cas had to work with, put up with. That they were the other being's family and Dean knew better than anyone how difficult it was to stray from a command given by someone you'd once believed to hold all the right cards. Dean knew what it was like to look the other way while your brother went around doing dumb shit because you wanted to convince yourself of ... a lot of things. Unlike for Cas; however, in Dean's case neither John nor Sam for all their emotional power over Dean could hurt him the way that he suspected that Zach could hurt Cas. The Winchester knew a bit about Zach's messed up ways of persuasion and lessons.
Dean was very grateful and one of the ways he hoped to show that was by not letting his own moronic brother bring about the end of the world. Making Cas' actions being something done in vain.
He, of course, wasn't surprised when the angels sent someone to stop them from reaching Sam; however, because his goal was Sam he didn't concentrate on that. His focus was on getting to his brother on time.
Dean was all kinds of grateful to Cas tonight. Traveling by Angel-Express had never been his favored way but he couldn't help but spare the tiniest moment of appreciate for it. Really. When this was all over he might even get all sentimental and by him a card. Take him to Hooters to celebrate. Because they had to. Dean refused to consider any other option, refused to give into the sense of failure already settling around the moment. The situation bleak. Because even if he did get to Sam on time. How would even begin to reason with his brother?
Dean had anger and fear and a million different forms of self-recriminations that twisted a sick not on his stomach; nevertheless, he refused to panic and his mind. His mind seemed to freeze onto the task. That is only until the scream seemed to resound through and out of the Convent.