Question: The people in my writers group complain that my plots have "too many coincidences." What's wrong with a coincidence? They happen all the time.
Answer: In real life they happen all the time, it's true. But Fiction is a compressed world. There, everything has to be cause and effect. So usually, coincidences are a no-no for fiction writers.
If you're going to have a coincidence happen, try to make sure it is foreshadowed by previous action so that the coincidence will be believable. For instance say two people are going to meet by accident (okay I can't resist a pun) in a fender-bender at a certain intersection.
Although fender-benders happen all the time, the reader might feel their meeting was too coincidental. But you could foreshadow that by saying having the driver see the upcoming light and remember that the traffic light there was malfunctioning last week. Then the possibility of a fender-bender (the next thing that happens) there becomes more likely. It's expected, not a shock.