9 seasons ago, we were introduced to Castiel. And now I wonder, why exactly did Dean try to convince Castiel?
He tried again and again to talk Cas out of his conviction towards the angels. He even prayed.
He never (!) did that with any other angel as far as I can recall. Perhaps because he felt like they wouldn't give up their past lives for humanity.
But he knew that Castiel had already risked that. Or maybe he was so passionate about Cas, because he felt like they had a connection?!
Or he noticed his doubt.
But whatever reason Dean may have had, he did it because he wanted Castiel to do the right thing.
Sure, having an angel on your side helps a ton in terms of the apocalypse and Uriel and basically everything else. But we know that it was more than that. How?
Well, if that all didn't already indicate that Dean felt like Cas was special, then the way he cared about him surely did.
I think Dean learned to hate mind control. We see that when he finally realizes, that his dad was basically using him all the time.
He wants to get Castiel away from the angels, because they are stopping him from becoming him who he really is.
And the whole season 6 plot still breaks my heart. Like, Cas was just trying to do the right thing. And it may have been obvious that it wasn't going to have a good outcome, but Cas kept believing that he was going to do the right thing for once.
And Dean refused to believe that Cas would do something behind their backs. ("...with every instinct telling him not to.")
Do you know what overpowers instincts? Feelings. Like love for example. Just sayin'.