So then irony of the statement is that he just described what it is to have friends, a best friend is a perfectly valid concept and language. It's just a reference to the friend who holds the most keys and the keys you view as holding the most value. It's literally that simple.

Acknowledging that certain aspects of life have greater value to you than others is one of the most authentic and honest things you can acknowledge in yourself. And recognizing that not everyone has every key and yet some people hold more keys and more value to you than others isnt outdated or immature, again its authentic and honest.

It's alright to have a best friend, you almost certainly have one, you are just too caught up in this nonsense to acknowledge it.