Afterwardsness and increased significance
When dealing with Fear of Flying, the fear is never really about the aircraft, or the actual flying. Most people with Fear of Flying only get the problem after they have flown many times. What happens is that something happens on a particular flight that frightens you. It might be sudden turbulence, or engine noise, or even something disturbing that you read on the flight. Whatever the cause, your unconscious mind searches for something like that feeling. You are searching for an older feeling so that your mind can know what to do immediately. But instead of finding a solution, your memory finds an old fear that was never dealt with. This intensifies the original fear, and links that original fear to flying. The psychology term for this is Afterwardsness.
Increasing that old fear
Seeking for a matching memory sometimes increases the perceived importance or intensity of that original memory. If you are afraid of flying, then every time you re-activate the memory trace, you also make the original memory more frightening. So your current fear of flying becomes more frightening. This process feeds on itself until it can become quite overwhelming.
This mechanism has been known for a long time. Sigmund Freud called it Nachträglichkeit. In English this becomes 'afterwardsness' or 'deferred action'. He refers to it many times when discussing the theory of psychoanalysis. He believed that memory traces are revised after the fact in response to fresh experiences. The second event gives extra meaning to original event. It becomes more meaningful than it originally was.
The problem of course is that you cannot consciously recall the original event. So you are left puzzling why something today is giving you severe anxiety. This explains the underlying cause of most phobias.
In a wider sense, every time you think about something it gets stronger.