At one time the word “hero” was simply a designation given to anybody born from the union of mortal and God, but that was in simpler times when the Gods would routinely roam the earth, ravishing comely mortal maids, and scattering progeny everywhere. With the coming of monotheism, that just wouldn't do. God singular couldn't be seen getting it off the local tavern keeper's daughter and begetting heroes all over the place. So instead of actually being the child of a God, a hero became a mortal who was able to perform tasks that required abilities beyond what is considered normal for humans.
Ever since, we've been inundated with tales of heroic deeds by men and women of valour, honour, and good breeding. It wasn't until the nineteenth century that anybody dared write a story with a person of lower status as the hero, and even then The Barber Of Seville was greeted with near riot when it was first performed, as the concept was considered so outrageous. Today it's quite acceptable for anybody to be a hero. While in terms of literature that's a good thing, when it comes to the reality of its application, the word has been diminished through overuse and abuse.
From sports "heroes" to supposed icons of society, it's become a convenient appellant to use for propaganda and marketing. While firefighters and other folk do things on a daily basis that most of us would blanch at, they are only recognized when politicians are looking to score points or whip up sentimental support for their policies. Most of the time we still associate heroism with the ability to kill people as shown by how many decorations and medals are reserved for those who are in the military.
Martial mien, as a standard for heroic behaviour is as old as humanity, and our earliest tales and epics all deal with the exploits of men at war or quests that bring them into conflict. In fact the oldest tale written down in the English language, Beowulf is about a warrior hero and his deeds on the battlefield. Perhaps because it was the first, and all tales in English since owe it a debt of gratitude, it has managed to hold on to our imaginations where others have failed.