Good list Whirly, to which I would add 'they scared themselves even while current and kept it quiet' (sort of like your number 10, but not quite)
Also people do get bored (flying is not all excitement) and like to collect additional ratings to keep their interest up. Some regulatory regimes make it easier to do this than others, and some regimes encourage them to do so more than others.
You also never see some people again after they have first soloed, sort of like your number 1, but without getting the PP rating. Seems fair enough to me.
Finding a good club is important, some places can put one off flying. Clubs in the UK tend to be less egalitarian than in other places.