Its phenomenal, how Humans are quite complicated creatures to study.
Your best Friend could be a ticking time bomb; waiting to turn a Stranger when certain conditions happen. And the other way round.

You’re happy with someone in the morning. The tide could easily change in the noon. And it’s the other way round too, because the Emotional framework of the Human Mind and its Bio System works intermittently; but like a swinging pendulum.

This phenomenon may have inspired an ancient Story, told in the very old Greece.. now attributed to Aesop. 

Source: Aesops Fables.

The story goes like this;
A certain Man and a Satyr happened upon friendship and decided to live together. They were in harmony, until one day in Winter-time the Satyr saw the Man blowing on his hands .
“Why do you do that?” He asked.
“To warm my hands,” said the Man.
That same day, when they sat down to supper together, they each had a steaming hot bowl of porridge, and the Man raised his bowl to his mouth and blew on it.
“Why do you do that?” Asked the Satyr.
“To cool my porridge,” said the Man.
The Satyr got up from the table.
“Good-bye, “ said he,
“I’m going: I can’t be friends with a man who blows hot and cold with the same breath.”

P.S: Aesops Fables tell of Humans and their Characters and how the Interact with themselves and their Environment.