'Dallas' hit a chord back in the late Seventies and Eighties because it was the age of greed: here you have this unapologetic character who is mean and nasty and ruthless and does it all with an evil grin. I think people related to JR back then because we all have someone we know exactly like him. Everyone in the world knows a JR.
A person who doubts himself is like a man who would enlist in the ranks of his enemies and bear arms agains himself. He makes his failure certain by himself being the first person to be convinced of it.