What is friendship? 

The friends are family that you choose our own self.  Our society tends to place an emphasis on romantic relationships.  We think that finding a right person will make us happy and fulfilled. Friendship has two most important aspects interdependence and voluntary participation.

Friendships require reciprocity of admiration,respect,trust, emotional and instrumental support.

Friendships require reciprocity of  admiration,respect,trust, emotional and instrumental support. Dr. Suzanne Degges White

Friendship raised from having same interest of hobby or they are same stage of life. Friendship does not happen, number of qualities and physique there will be same.
Best friends improve your mood,spending a much time with you. Spread a smile on your face.
They help you to reach your goals. They motivate and boost your interest and skills,while you are lazy your own self. 
They support you time to time in achieving your targets, sharing problems to solve it,friends can help you cope with serious illness. They are with you when the loss of job, breakup of relationships or any kind of challenges in your life.
Friends do not disconnect as you become aged, they are alway in your company even you are in depression or stress. They help each other as they can. Up and down are the part of every relation but they come together very soon even they fought too hard nearly. Forgiving, helping, teasing, motivating are the basic points which make them close together.
Now a days social media change the meaning of friendship,they have introduced different social websites and link them at a single platform but this is not even like that to click a Add Friend button and become a friend forever. At these websites friends call each other for their need. 
Friendships require reciprocity of  admiration,respect,trust, emotional and instrumental support. Dr. Suzanne Degges White

Last but not least is the truly few and far between phenomenon of the "best friend." "Best friends are the rarest type of friend and the kind of friend that we all need to have in our lives. It's the friend who gets you without you having to explain yourself. It's the type of friend who loves you no matter what," Saeed Ali says. And they're not necessarily people you talk to every day. "You might go weeks or months without connecting, but when you do re-connect, it's as if no time has passed at all. These friendships are different in their ability to flex and endure even if life temporarily gets in the way. These are 'heart-to-heart' or 'soul friends' and they can give you comfort even if you're out of touch with this friend."