When does and aquantance become a friend?

This is something that has been puzzling me recently?

I have lots of acquaintances, I believe, but few friends.

Is a friend, an acquaintance  who you can be unreasonably nasty to and still comes back for more.

Someone you can tease about their sensitive areas?

Someone who drops in regularly unannounced and vice verse?

How would you describe a friend?

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