I kept on thinking about your line " there are no coincidences - "coincidences" happen for a reason", and that evening 27 years ago when I sat down to read The Unbearable Lightness of Being and when I got to chapter 11 the phone rang and it was Sandra telling me that Antonella, whose favourite book was The Unbearable Lightness of Being, had finally been found - this time as a heroin drug addict desperate for redemption.

Coincidences.

They happen for a reason, you say.

For many years, I have been keen on picking them up whenever they took place, but always thought of them as something bizarre, perhaps magic, certainly cute. I never thought they would happen for a reason. But the other day I thought about your line. I remembered something else, a paragraph I had copied from a long-forgotten book, twenty years ago. "What if we are just characters in a story, what if what we perceive as our existence is just a narrative? How could we possibly know? Assuming the story is coherent and the characters are meaningful, only an external event would bring the character's attention to the type of story he is part of."

Our lives are stories.

Our lives are stories, and the coincidences are the tools to understand their plot. Or perhaps they are the signposts between the different chapters - you understand that there is a shift in the novel only because the new chapter is marked. 

Daniel Pennac's The Dreamer's Law. He's 60. He goes back to the artificial lake where he had been swimming as a child, one morning after a vivid dream he still remembers. He goes to the lake and dives in and realizes that the water covers a small town, flooded 100 years before, and the submerged town looks exactly like the one he had dreamt as a child. He swims back up and out of the water, his best friend is waiting for him. Not on the lakeshore but in the hotel room they were sleeping. "There is no town under that lake, and you just had another dream," he says.
Chapter 8. The Dreamer's Law.
"It is then, in the middle of life, that there large screens for the dream."
Fernando Pessoa, "The Book of Disquiet"