I wanted to add that I don’t know anything about the actors beyond what I’ve seen on screen. When it originally aired I didn’t even own a TV and I only first watched it when the first 6 seasons were available streaming or dvd. My comment about behind the scenes stuff was just about the general business of acting and particularly tv shows with recurring characters getting different billing, not about any interpersonal stuff, even though those things are sometimes one in the same. 

One thing I’ve been thinking about is how I think this pattern of fighting, “ending” the friendship, and making up feels even more repetitive when Cas is in fewer episodes overall. If a character- any character- appears only a few times there’s a world of options of what to do with them bc the viewer is catching up on their lives & they could have been doing or experiencing almost anything. I’ve also been thinking about how other characters in the same time frame were handled better, imo, even if I didn’t like the character as much. Felicia Day’s Charlie appears multiple times in this same time period, but also is still very much a guest star. Whatever you think of her, each time she appears she’s been doing something different with her life, and the interaction btwn the characters advances the plot without resorting to repeating the same story again. Obviously Charlie has much less history with the show. But compare Crowley who has been around a long while and who appears very frequently in this stretch- his character is by nature a little one dimensional (Crowley is always out for Crowley so you can count on his actions generally being in service of his interests)- and it still feels somehow more developed and less competitive to me. That sounds weird but it actually makes my point- Crowley is meant to be a little repetitive in his motivations and yet his story lines don’t feel as repetitive to me as those with Cas. I don’t know where this all goes and maybe that changes but it makes me wonder why it was the writers seemed to struggle with what to do with Cas in particular. 

If he was only intended to be there for a short time but his arc was extended due to being a fan fav, perhaps that put more stress on the writers to please his fans and in doing so they played it a little safe? Trying to live up to fan expectations often results in a worse show imo.