“Knowing you, it… it’s been the best part of my life"
Here’s what gets me about this line. It’s the first time I can think of that an angel talks about having a “life” in human terms. I mean, these are 4 billion year old, immortal creatures. The concept of living a “life” just doesn’t make a whole lot of sense if you’ve been around forever and plan to be around for the rest of forever. Angels don’t live and die, they just exist. So, Cas talking about how the Winchesters are the best part of “life”–this is more than Cas saying how important the Dean and Sam have become to him. It’s a whole radical shift in how an angel conceptualizes his very existence. Which makes this, to me, a way more monumental moment in Cas’s arc than his calling the Winchesters family or even saying he loves them, concepts that the angels already use to understand and organize their relationships. Nope, this is something different, something new. The Winchesters made Cas more human. More than that, they gave Cas a life.
(And I’m not making this all about Destiel but it totally could be. Of course Cas describes Dean as his family–angels only relate to each other in terms of family, and all Dean talks about is family, so what else would he call him? But this is a direct acknowledgement that his experiences, his “profound bond” with Dean, are meaningful in a whole other way. Shipping goggles aside, though, I still think this is one of the most important lines in Cas’s entire arc.)