Dean has been trying to be open with Cas, telling him he's worried, we're better together and Cas doesn't really respond except with what could be called flippancy saying I didn't mean to add to your burden or betrays him again. So it's not that Dean needs to open his mouth, it's that when he does, he's shut down. This happens a lot in the series with other people too and I think we undervalue how much that shapes Dean's willingness to open up. As in, I think we often ignore it completely.
This is an interesting point, and the thing is, we’ve come so far in this ‘haha, we’re not describing a relationship, #no homo’ thing that the tropes of the genre have started to have an impact on the actual narrative of the show. Like, one of the main reason I don’t like romance movies is the lack of clear communication between the two main characters. I mostly hate it in its classic ‘chick flick’ form - ie, a woman going on and on with her girlfriends about how perfect this guy is and being encouraged to do all sort of ridiculous things to catch his attention rather than just talk to him - but if the movie is not subtle, I’m even capable of hating those pregnant ‘I wish I could come out and say it, but I’m too manly to’ pauses because, come on. Sometimes I truly find romance movies are the plague of our societies and are way, way more dangerous than horror or violence or whatever, simply because they set a model of behaviour for situations we actually encounter IRL - and the idea that we can’t communicate openly and honestly with each other, especially in a romantic relationship, is often at the centre of whatever demented story they’re telling.
I guess this is to say that miscommunicaton is a classic romantic trope and the most usual way to keep lovers apart (short of, let’s say, family obligations, mind control spells and terminal illnesses - not that Supernatural has used any of those, of course), which means neither Dean nor Cas are, at this point, to blame for any of it. It’s simply how the narrative goes, and one of the most tried-and-tested ways to preserve some semblance of UST and will they won’t they even in those circumstances where it’s very clear that yeah, they will.
You say Dean’s been opening up, has been clear about what he wants, and that’s true. Dean’s been incredibly direct this season (and the last), not only with Cas, but with Sam and Mary as well. I think it’s unfair, though, to say we ignore it - some days, it seems we talk about little else: performing!Dean walls coming down, that’s a huge bout of character development, especially considering Dean’s worst fear is people leaving him, and, well - if you don’t come clean to someone, if you don’t tell them how much they mean to you, then you can hold on to the illusion they left because they didn’t realize how much they would hurt you - but if you’re clear about your feelings and you do your best and they still leave - ouch. Dean’s faced this dilemma with both Mary and Cas this season, and basically lost both times. His ideas of creating some kind of patchwork family - all of them safe, happy, and living right there in the Bunker where Dean can keep an eye on them and protect them - yeah, that didn’t work. Despite the unusually honest conversations he had with them, both Mary and Cas continue to do their own thing without much regard for Dean’s feelings - and I feel like I need to stress it’s not only a character’s ‘fault’, but a narrative need: to keep Dean on edge, and to make him miserable.
Something else we need to consider is that Cas is new to humanity and sees things from a completely different perspective. To him, human feelings are - well, not irrelevant, but I think he sees them as something so complicated and changeable that it’s not really worth analyzing them. Like, consider his impatience when Dean asked for help in how to deal with Mary - Cas is millions of years old. Planning a conversation, however life-changing that conversation may feel in the moment, is completely irrelevant on the long term - a speck of meaning against the backdrop of eternity, or even of a human life. When he shuts Dean down, he’s not saying he doesn’t care about Dean feelings; he’s saying everything will sort itself out, and why do humans always stress so much about such small and transient things? It makes no sense. Oh, and another thing that presumably makes no sense to him is Dean’s worry over him - Cas was created to obey and walk into battle and die, if necessary - nobody’s worried about his happiness or wellbeing for thousands of years - why would Dean? Cas is an angel - Cas is the (self-appointed) Winchester’s guardian - not the other way around. I know they keep telling him they care (sort of), but, again, different species here. And Cas also knows, because he’s seen it, that (unlike angels) humans are built to withstand grief and loss - that both Dean and Sam have done it, several times (that despite everything they’ve endured, they can still be sort of happy). To him, his own death is perhaps a matter of regret, of things unfinished, but certainly not something that’ll hurt either Sam or Dean in any definitive way - which is why he prefers to be out on the field and keep them safe, rather than staying close to them and avoid danger.
Finally, something about the mixtape scene.