No birth certificate is issued when friendship is born. There is nothing tangible. There is just a feeling that your life is different and that your capacity to love and care has miraculously been enlarged with out any effort on your part. It's like having a tiny apartment and somebody moves in with you. But instead of becoming cramped and crowded, the space expands, and you discover rooms you never knew you had until your friend moved in with you.
One of the surest evidences of friendship that one individual can display to another is telling him gently of a fault. If any other can excel it, it is listening to such a disclosure with gratitude, and amending the error.