The documentary is really good. Explains that his family don't really talk about their feelings, so they go into his songs. Probably why you cry pretty much every reaction to him. On another note, have you ever listened/reacted to Louis Dunford? Check out 'The Boy Who Could Fly' or 'The Ballad of Benjamin'.
I think at least one line of this song addresses the imbalance between couples who have one side more famous than the other, or more powerful, perhaps. Imagine James Blunt going to a dinner to honour his wife but everyone is dazzled by James and ignores her. Something like that would eat at you over time. Now, I don't know that this is really happening, okay? It could be any famous couple, like Julia Roberts and her husband Daniel. Or George Clooney and his wife Amal. Dolly Parton's husband simply stays out of the limelight because that is the way he prefers it.
