There was an interesting thread on the flying instructors forum a few months back, about when it was time to tell a student that they just weren't cut out for flying.
Some of the stories focussed on students who, although were probably technically and mentally capable of flying, had personality traits that made them unsuitable : they flipped out screaming and shouting at instructors when criticized, or got very stressed or froze when the pressure started.
I'm sure these people can be taught to fly, but not by just by a flying instructor - they need professional, possibly clinical help with the personality trait that makes them unsuitable. I think there's a lot of drivers on the roads today that the same could be said for.
I'd disagree, by the way, that flying "is as easy as" driving - something that I've seen quoted a lot. I think it's quite different, with different skills and traits being important. I think, however, that if you can be taught to drive, then you can probably be taught to fly.
Ah - here's that thread :
http://www.pprune.org/forums/showthr...t=shouldnt+fly