I think communication is more important than it is detrimental, but it is potentially detrimental too, or else why would people avoid it sometimes? Revealing your true feelings is all well and good, but those true feelings might be hurtful to the other person. For instance, while hearing Seishin's real feelings (canon feelings such as his own personal view of justice, let alone our interpretations of his resentfulness) Ozaki might get kind of freaked out and maybe label him as crazy.

Although not to that degree because they've been friends long enough for him to accept Seishin with all of his quirks, but he might just be unable to understand; "Why would anyone waste their time contemplating that kind of stuff when this outbreak is happening? That's way too much work." Or he would see Seishin as fragile and keep things from him because he doesn't want him to become even more dark.

And if Ozaki revealed his own feelings, say telling Seishin that he believes all the Shiki must die to save the village as a whole (that's basically what he does in their breakup scene), that would be so against Seishin's morals that Seishin might just leave him then and there.

And as a real-life example, if you have negative feelings towards someone, it's best to just keep quiet, right? One could confront them, but that might not do much good because it probably won't do any good and now they'll think you're crazy...that's why people usually are civil to those they dislike, also to avoid coming off as an unpleasant person. Maybe that's just me though, I'm not very confrontational.

Anyway I'm just thinking that Seishin is afraid of Ozaki judging him for the reasons I specified, which is why I think he doesn't reach out. And like you said, he can't even interpret his own suicide attempt, so how would he expect Ozaki to take him seriously? Plus, another theory is that Seishin just hangs out with Ozaki out of inertia and holds him in less esteem than he deserves. Although Seishin admires him, he probably underestimates the depth of feelings Ozaki might have toward Seishin. Ozaki considers Seishin his best friend, and probably hangs out with him because he likes him, not because he's used to it. But simply because their personalities are so different, Seishin may interpret it as not caring--and I think he could interpret Ozaki correctly, but he's kind of preoccupied with his own existential crisis, poor guy!

Ah, I think I can analyze a little more about he little brother! I get why the little brother died, but I was also wondering why he continued to pursue the older brother as a Shiki. I think it was either because the little brother is a memory of the dead facade, or if it was because there was a ghost of a facade left. To elaborate, after Seishin's public persona is cast away, he must still remember what kind of a strain it was like to see it there. So he might have some regrets, such as "would I have been a better person as the perfect priest"? So it still follows him like a shadow...and like a Shiki, it comes out at night because night is when people get all their profound thoughts. Or, maybe it's because the facade still lingers. I think when Seishin kills it is when he leaves Ozaki in the graveyard and goes to Kanemasa. There, he's still gentle and indecisive. He's passive, and he still needs Sunako's help to interpret his attempted suicide. After they've done that, and after she's gotten threatened, is when he casts off the remains of the persona and does what he wants for the first time in his life: he wants Sunako to be by his side, so he saves her. It's ironic how the new Seishin, the older brother, is a jinrou, while the little brother who is supposed to be a Shiki, was who the human Seishin was.

If you were to develop a crush on Ozaki, you wouldn't be the only one by a long shot!