Who is your best friend?

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I responded to a discussion about a best friend turning to an enemy,and i ask myself why did he think that he is his best friend? How did he you choose his best friend? what are the characteristic of a best friend? I believe if these questions are answered i might be able to know if my best friend are thoroughly my best indeed. How do you know someone is your best friend? And if you are betrayed by someone you consider your best friend will you be surprise?