I'm angry at my brother, and I don't support his decision to quit his job.
What makes you react that way?
Because he's such a moron for doing such a stupid thing, I was hoping he'd take over our family business.
It's reasonable that would be angry with a family member who makes a rash decision, especially if you were counting on them.  However, your brother should be free to make his own decisions about his life and his career, and you should ultimately be supportive of him.
Yeah, you're right. I don't have to tell him what to do.
You don't need to agree with him, but he can decide for himself.