M.H.
Hi C.,
I bet she is a very concrete thinker. She's very loyal and feels compelled to be a perfect friend even though her friend is not giving as much to the relationship. I know this because it sounds like me when I was her age. I had other friendly acquantances but only one "best" friend at a time.
On a positive note. This personality trait goes FAR in a marriage. You said you have a close nit family. You have taught her loyalty and closeness with someone she trusts. Your relationship with your parents encourages this also. It's true that it is painful now but this will take her sooo far in life and she will be respected as an adult. She is probably right that there is no one that is worthy of her friendship because she knows what a real friend is supposed to be. (Your fault again, lol) Not many people do. My sequential "best" friends in my childhood have numerous acquaintances now and not many of them have true friends or a successful home.
I really wouldn't worry about her. BUT you are correct in encouraging her to be friendly with others. Opening up to someone is truly something that is reserved for someone you can trust. That takes time and it also takes proof.
Show her my letter if you like. I felt like you were talking about me for a split second....
God bless!
M.