We often use "Uncle" to refer to a paternalistic figure or close family friend who isn't actually related by blood or marriage. For example, I'm a godfather to the young children of a family friend, and I'm usually introduced as "Uncle John" to the kids at birthdays or holiday gatherings.
It struck me that this is rather arbitrary.
Why isn't it, say, "Brother John" or "Grandpa John" or "Cousin John" or any other similar term?
Does this have any origins in "Uncle Sam", the national personification commonly used in America? That's the only connection I could think of.
Is this sort of thing common in other languages/cultures, too?