When you don't want to, can't, or it wouldn't be prudent to claim to have only one best friend. You have plane friends. They are best friends that you would jump on a plane for if they said they needed you.
Choose your plane friends carefully because they would also have to be willing to do the same for you. Generally, you don't have these until you are an adult and all your friends are scattered throughout the country.
I don't see Marc that much, but his still one of my plane friends.