Perhaps you are familiar with 2 quite famous experiments in psychology: the Stanford Prison Experiment, and the Millgram experiment. If not, the summary is that when people are cast in a role, they change their behaviour to be more congruent with the behaviours expected of that role. A film was made based on the Stanford Prison Experiment that is well worth watching (imdb=7.9), called "Das Experiment" (German - make sure you download the English subtitled version!).
This effect is part of what is known as Attribution Theory in psychology. An everyday example of the application of this was discussed briefly in my post on persuasion - calling someone "mate" or "buddy" will help elicit trusting and loyal behaviour.
The effect also must be borne in mind when explaining how whole groups of people performed acts considered 'evil' such as all the Nazi guards herding the Jews to their deaths. Humans seem to readily accept that there are psychological reasons excusing bad behaviour, but conveniently forget the other side of the mirror, that the same forces are at play in the other direction as well.
Soldiers, police, and firefighters are all called 'Heroes' if they go into a dangerous situation and get killed. It is much more often when they are killed, possibly because you can't speak ill of the dead - so no-one is allowed to bring any evidence forward opposing the moniker.
After the 9/11 attacks the firemen (and women) who responded were called heroes. I think most of the population, if they had worked in the fireservice for long enough to hold it close to their personal identity, would have done the same thing - in which case are most of the population heroes who just haven't had the circumstances to demonstrate it?
I think it's great people like that are publicly called heroes - they put themselves in harms way for my benefit, and if we didn't impose the expectation of them being heroes perhaps they wouldn't. It is in society's interests to call them heroes (and mean it!) - and that is why that meme has evolved.
Sure, some may join up as a vocation, but I think most are probably sheep just acting out a role thrust upon them.