One important thing to know about Kazuki is that he doesn't like talking about his feelings. This can be REALLY ANNOYING when you consider how Kazuki also wears all his emotions on his sleeve; it's pretty easy to tell when he is upset, or fed up, or happy, etc.! So he's fairly honest about his feelings WHILE HE DOESN'T TALK ABOUT THEM.
In canon this creates a rather one-sided relationship with him and his people. Kazuki likes hearing about other people's feelings, because unlike him other people tend not to be so upfront that they're upset! e.g. Naoto. Just... Naoto.
However, on more than one occasion when people ask Kazuki about his feelings, such as why he's upset, Kazuki SHUTS THEM OUT. He doesn't want to get into the complicated whys and hows! He just IS. They know he is, they know it's honest! It's fairly easy to tell why if they pay attention to what's going on! And if Kazuki knows anything, he knows geninely upset feelings make things complicated, soooo he just... shuts himself away when they happen to get over himself. He is not going to be fake smiling and making everyone be at ease! When he says "It's fine", he is basically saying "It's fine, nothing can change how I feel!" rather than "It's fine, everything is okay." It gets to the point where one of them practically non-cons Kazuki into opening up about his feelings, and even then Kazuki NEVER tells the whole story. Kazuki is a lot more private than people give him credit for, and none of his friends knew what he was planning, or necessarily how he felt about it. They just knew he was very upset about SOMETHING, and made their own assumptions. To be fair, half of those assumptions were correct-- after all, there is context and what Kazuki DOES say about his feelings to clue them in. For instance, Kazuki gets so upset about people dying he keeps saying he doesn't want anyone else to die! HE IS REALLY UPSET AT THIS POINT IN CANON. What people don't know is that Kazuki is also terrified of dying himself, and what he know he needs to go through. In other words, it's not the full story. Because Kazuki doesn't tell 'full stories'. He tells what he thinks he needs to, what is other people's business to know, then moves on. Because he is not the type of person to dwell on complicated messes, nor does he want to create complicated messes.
Which is why in camp, Kazuki has been having trouble communicating with people and being upset lately! Because people keep wanting to emotionally connect with Kazuki, for him to tell them why he is upset. And Kazuki is doing his usual thing of shutting them out-- even, to a certain extent, Haruki. Haruki, one of the few people, if not the only one, Kazuki opens up to completely. Because Kazuki is a Fixer. He is there to help his best friend emotionally, no matter how he feels, to make things less complicated for everyone. And he thinks Haruki is much worse off than him at them moment, and all he wants to do is live a happy life with everyone. So, what's the point in being asked how he feels over and over when it's obvious from his actions that he is fixing things to feel better! He TELLS people he is fixing things to get better! What he doesn't say is WHY he feels he needs to fix things, except when particularly pushed-- which has been easier recently. But after the first few days, which were a BLUR of complicated emotions for him, Kazuki got himself together and is starting to "feel better", if by feel better we mean is trying to move on from his feeling feelings. And I do.
This especially is affecting his relationship with his other best friend Yuki. After all, even if they're best friend, Kazuki isn't exactly divulging his feelings here. The best Kazuki divulges his feelings to Yuki canonically is, in a round about way, telling Yuki he misses him over text. When Yuki asks for confirmation of feeling from Kazuki... while he doesn't wave them off, it LOOKS like he does because he honestly does not see the point in dwelling. And this is reflected in his actions! In camp, Kazuki decided not to tell Yuki about Yuki's death initially because:
a) Yuki just found out he died, at Haruki's hand
b) He hadn't quite come to terms with it himself. And he still hasn't!
But in a comedy of errors, Kazuki told Ruka about it because he thought Ruka could handle it, Yuki wasn't listening in, and he wasn't in the "right" emotional mind either! Ruka was... while not demanding answers, Kazuki, tired of dodging questions and thinking that his friends DID deserve to know what went down... decided to try giving divulging his feelings a shot! Because to Kazuki, other people's emotions, especially his friends, need to be satisfied more than his because Kazuki knows how to get over himself. And it backfired on him HORRIBLY, just making things more complicated and upset with his people, so he's just going to shut himself up again. Unfortunately, this makes it look like Kazuki is ignoring his feelings-- and okay, to a certain extent he is. But he also isn't, because he know that the only way to make himself feel better is to fix things. It's just that he doesn't realize, or refuses to realize, that people want to help him.
So when Yuki came along and wanted to connect to Kazuki emotionally and understand what Kazuki did, this... didn't go well, considering what we know. While they have made a 'peace' and for all intents and purposes returned to the status quo, Kazuki doesn't trust people to understand him emotionally, and in fact expects people just to make his feelings complicated when they are really quite simple. After all, he is wearing his emotions on his sleeve and doing what he can, and not even his BEST FRIEND gets it! So why bother trying to go through the exhaustive complicated process of talking out feelings again when it just makes things worse? The best Kazuki has done since then is mention it in an almost off hand way with Kida and Sera. Again, something that only fuels the problem he is having with Yuki-- Kazuki is learning to tell people his feelings in an off-hand, almost analytical way. It makes him look blase! Of course, because Kazuki is Kazuki, him getting a bit worked up when he tells people is always going to happen. But to him, that's the best way others can understand it and he can cope with it, and is not the full story. Everything else, he'll show. He's good at showing! But because he's so off-hand when he's telling, people such as Yuki aren't noticing what he's showing either-- that he's doing what he's doing, which is fixing Haruki and trying to be 'normal', because he can't handle any other options.
So Kazuki thinks this should be fairly obvious. Why should he cry over Yuki's death when Yuki is there, and they can be happy? Why would anyone want someone to go back to being so heartbroken at someone's death, over the betrayal of a friend for strange goals? Those are the sort of thought processes behind his 'fast' recovery, and he is tired of people doubting his emotions-- the very ones driving his actions. He isn't ignoring how he feels, he is heartbroken and he hurts. A LOT. Kazuki just... isnt' good at letting things go! Be it people or emotions. He knows this. But he is also showing he is strong enough to get through it despite his bad moods! He is not a smiler, when he says "it's fine", he is not saying everything is peachy. He is saying that is how it is, but it won't be forever. It's simple. What he's doing is simple. Why talk about it?
Kazuki, in other words, doesn't like displaying emotional vulnerability, but he also isn't emotionally dishonest. He quite easily displays the more 'positive' feelings of his personality, but when it comes to his darker feelings he needs to literally shut himself away for a while, or throw a temper tantrum, and do ANYTHING to express himself while not complicating the issue. Because he is an extremely easy person to read! And he KNOWS people can see it! It has just been frustrating to him that now people are demanding that he express his darker emotions more when he's... developed to the point of knowing that dwelling on such emotions don't do a damn thing, and it's better to use them to drive for something positive. Anything else is, again, making things complicated. And it HAS got complicated, even with Yuki- his best friend. And the only way he knows how to fix it is to do what he does best: Simple, everyday, things.
Ask me things in comments because I'm sure I missed a lot of stuff.