Actually it may have little to do with luck. People tend not to be attracted to people with whom they spend lots of time before the age of 6. This behaviour evolved to decrease the chance of inbreeding, however it does not only apply to family members. If the OP has known their best friend since before the age of 6, it is likely not luck, but genetic behaviour that has kept the friendship as it is.
Is It Normal To Have A Girl Best Friend
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Actually it may have little to do with luck. People tend not to be attracted to people with whom they spend lots of time before the age of 6. This behaviour evolved to decrease the chance of inbreeding, however it does not only apply to family members. If the OP has known their best friend since before the age of 6, it is likely not luck, but genetic behaviour that has kept the friendship as it is.