So I DO get angry when they act immature or childish IF the story focuses so much on the whole "adult in a child's body" aspect. BUT, if they just go adult died, reincarnated, and now they're a kid again and go with it I don't care. I've seen it done well even where the MC is like "I shouldn't let this get to me, as I'm an adult," and then fail to do just that and their physical age shows, and it's wonderful. Like, the understand it's wrong but still act out (as a child does). Most the time though it's done horribly with the reincarnated character claiming to be an "adult" and "mature" and yet they act WORSE than the "legit children"... it's frustrating. So yeah, I'm glad this manga actually addresses it and gives a reason for why he loses control over his emotions at times.
I must say, I'm SO glad that one of these reincarnation mangas FINALLY addressed the fact that even if their "soul" is an adult their "mind and body" is that of a CHILD! A child's brain isn't like an adults brain. Thank you, thank you, thank you for finally addressing this!