What the hell is the black haired guy’s problem?

i think it’s because last chapter he was upset about him going out and socializing… thinking he’s just being insecure ┗( T﹏T )┛

Also, in the last chapter the black-haired mentioned the pi j hoodie. He was upset that he found the top wearing it with his friends (the girl's insta). That's how he got the top in the first place.