What is Best Friend

Sometime i so confuse becuz i dun know wat is best friend. Why everybody need a best friend,now i know best friend is a friend tat know u tat always care about you that always share wit u even u sad or happy.But i still dun know why other people best friend always be good to each other always care about each other but why my best friend diffrent. I think we not the best friend cause u dun know the word "BEST"