So, Josuke. I’ve touched on this before, but developing Crazy Diamond at such a young age (4 years old) seems to have permanently altered Josuke’s perception of cause-and-effect, particularly as it pertains to pain in others. He has a particularly skewed form of empathy; not that he is not empathetic, but that it is perhaps dulled.
He’s spent ¾ths of his life being able to heal people and things, which, at his age, is definitely long enough to have changed the way he thinks about and reacts to injury. He’s said it himself: he fixed broken bones in others all the time. Things that might be a traumatic experience to others are something he can make vanish with a snap of his fingers. While this is undoubtedly handy, it also means he’d have a hard time processing the severity of an injury’s effect emotionally or mentally on others. Sure, he can’t heal himself, but that also means he’s unaware that trauma doesn’t disappear with the wound.
As such, he’s become numb to physical harm. He punches through two people in a gruesome manner, one of them being his own mother, and heals them up in a flash without so much as blinking. He doesn’t seem to be disturbed by his own actions because he knows he can heal them later, disregarding the fact that he just punched a bloody, gaping hole through another person. When Okuyasu’s arm is cut off during the Red Hot Chili Peppers arc, instead of reacting with any sense of urgency, panic, or any other emotion you might associate with having your best friend lose his arm and be dragged away by the enemy, he completely underreacts, viewing it as an advantageous thing in the long run, and is completely unconcerned with the severity of the injury and situation because he can just stitch Oku up in a flash.
It’s probably for this reason that Ryohei’s death affected him so much. It’s not just because he was family. It’s not just because it was a side effect of Josuke not paying attention. This is the first thing he could not fix. He healed everything, he was there as soon as possible. Death by injury is as impossible for Josuke to understand as Stands are to non-Stand users. It’s not just that he was in denial; another person dying of injury, an injury that he could not fix, was quite literally impossible for him to understand. His brain could not register it as even being a fact.