The frustrating thing for me, when people use Dean's issues to point out WHY he handles emotional situations badly is that the sympathy isn't with the people that suffers because of it (like Cas). It's with Dean. An explanation for Dean's behaviour shouldn't become a justification. If Dean loves Cas but treats Cas badly, why should we root for this relationship? Him almost killing Cas is about CAS' suffering, not his. But his reaction is not about what Cas deserves, but about his wants. IMO etc.
I’ve been trying to come up with something clever about this ask for a few days, but I got nothing, so I’m just going to tell you what’s going through my mind in the clearest possible way.
1) The viewers will often have a favourite and defend them to the death, and if you find it frustrating, there’s not much you can do other than unfollow people or blacklist some tags. I understand where you come from - I also get annoyed when people justify everything’s a character’s ever done, even the most twisted and problematic things, just because they like him or her. To be honest, I think I stopped caring when I saw a discussion about how Tate Langdon was the perfect boyfriend - some part of my soul just shrivelled and died and I decided that yep, some people are batshit insane and most people get unreasonable around stuff they love, and what can you do about it?
2) If you’re talking about the writers/creators of a show, on the other hand, I think it’s important to remember where is our POV and what kind of story those people are telling. Like, Supernatural is not House of Cards: whatever he does, Dean will be written as sympathetic, and since we see this world (mostly) from his POV, everything is reflected back on him. I know some people get angry about this - characters getting killed to make the Winchesters feel bad, or simply to advance the plot - but that’s how you tell a story (everybody does the same thing, and if you don’t see it, it means they’re doing their job right). Your main characters are the ones who matter, and the ones whose emotions we care about. So, even when it comes to someone as important to the story as Cas is, Dean will get most of the plot, and this is just how things work.
The next point might upset some people, so I’ll place it under a cut. Stay safe.