Dean's fear of flying isn't your typical run of the mill discomfort. Where did this fear come from?
Write a fic that explores Dean's fear of flying. It could anything from an accident on a hang glider, zip line or hot air balloon to an actual plane crash.
Sammy either can't be there when the injury occurs (since he doesn't know about the fear) or is too young to remember the incident.
Since Dean states in Phantom Traveller "Quit treating me like I'm four" Kudos if the event occurs when Dean IS four as I'm convinced that's when his fear initiated. But feel free to make it start at any age.