why is rowdy mean to junior

Rowdy is Arnold's best friend but he also pushes Arnold around. Their friendship is a paradox of sorts. Sometimes Rowdy's only way of releasing tension and communicating is violence so sometimes, like in an abusive relationship, Arnold gets hurt,

I was the kind of idiot that got punched hard in the face by his best friend.

Bang! Rowdy punched me.

Bang! I hit the ground.

Bang! My nose bled like a firework.