I read a statistic some time ago that most people change careers on average 3 times during a work life. The idea that we choose an occupation in teens and work a dreary career until we are 65 is not the case in our society. Pilots seem to come and go mostly because of the fickleness of aviation, but most people change careers because they have become bored and are not challenged. We see ourselves doing something based on our needs at a particular time and as times goes by those needs change. It is a wonderful thing that our society allows movement like it does and this helps keep enthusiasm high. Freedom in choosing careers is something associated with affluent, progressive cultures. You do not see it as much in say China, India, and the former Soviet Union.