Why do you suppose Joey Costello acted the way he did at tangerine middle school?

When Tino and his gang teased him, he got mad and "fell for the bait." Paul let Tino tease him, calling him names. Paul didn't mind, so he got accepted into their group. Joey started to act hostile and mad, so he didn't get in their group, causing him to act different, and think he is superior to everyone else.

But the reason Paul didn't mind was he had an older brother, that was really bad to him. He was much worse than any other brothers. While Joey had a sweet older brother, and wasn't used to being taunted. Plus, he was really composed after Mike died (for his age anyway) and he had to find a way to vent somehow... so he took the taunting a little more to heart, because he was really sensitive.