Kazu is closing himself off of from the rest of the world, and Jun has this 'thing' with Satoshi.
*sighs* Teens.

I guess in a way, Jun realizes that Satoshi was just too worried over Kazu, and hence the overreaction. And as per Masaki says, they both are closed individuals, and when a problem occurred, it's hard to bring them back together. With Sho and Jun, they tend to burst out at each other. A huge catasthrope will be sure to follow, but at least they can make their points across. With Jun and Kazu, or Satoshi and Sho, they're fairly easier. One throws the words, the other listens. But Satoshi and Kazu? A silent war is what you'll mostly get. Well, not an actual war perhaps, because they somehow could understood each other enough when the fume fades. But both will forever be insecure as long as they don't speak it out clearly.
And Jun knows. He's just too young and overprotective. All he cares is that Satoshi is making it harder to his favourite brother, and he's intent in making him feel the same.

The visit to Sho's place is rather cute, if anything. But both Sho and Jun know that Jun needed it. He needs assurance, that he's not fighting along to keep Kazu at bay.
And his latest talks with Masaki is adorable~ Both of them are~ I like it when Masaki is being all flustered at being praised. I guess he's too used at his older brothers calling him irresponsible and all. Knowing that he's being loved the way he is sure meant a lot to him. And of course, that acceptance of dating Jun's Italian teacher must have been a relief, too LOL

Thank you for the update!
Can't wait to see how things will unfold from here on.