Why and how do we choose friends?

The famous writer CS Lewis said, "'What! You too? I thought I was the only one, 'we start friendship,' he said.
Likewise the Greek philosopher Plato said "similarities will bear friendships". In 360. Aristotle made a similar comment: "We are told that we like what we have."
Friendships glowing on the sharing of thoughts and pleasures seem instinctual; but this is deceptive. Most friendships develop outside family members and spouses. In other words, friendship can not be explained with genetics or in order to maintain the generation. On the contrary, evolutionist biologists base their friendship on mutual benefit. I mean, if you scrape my back, I will make you a scratch.
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But social psychologists have shown that people do not keep the gang of help they get from their friends and friends. Primatologist Joan Silk states: "Mutuality and equality are important among friends; but the understanding of the search for the truth of everything in the same way is contrary to the establishment and maintenance of close friendships. If this contradictory situation is true, friendship is a big puzzle for evolutionists. "