I see Greg as someone needing a father figure. I know a kid just like him. His dad is around and in his life, but he doesn't talk or show emotion. The kid is desperate for some feedback and one-on-one time with him. The kid walks around acting tough and talking back as a way to get attention.
That's what I see in Greg. I have a little theory that people act out because they are missing something in their lives. I think Greg tends to have outbursts because he is seeking attention that he doesn't get at home.
It's amazing how deeply painful something like that can be yet how common place it is. I suppose this is probably everyone's story to some degree.