I think it stems from the possibilities you have in school. At 14 - 16, you could still possibly become an astronaut or prime minister if you really set your mind to it. Of course nobody does, cause theres other things on your mind at that age. Then the realisation comes once you settle into a career path that your options are greatly limited and perhaps if only you'd tried a bit harder or took a different subject your life would be immeasurably better.