Imagine for a moment that you are, right now, a human of the same age that you are, but living in Azeroth. Depending how old you are, you either lived through or were born into the aftermath of three of the most devastating wars your world has ever seen. Keeping in mind the trouble with timelines, every human alive in the Warcraft setting has endured loss and hardship on a scale almost unimaginable; many were driven from their homes by invading monsters or demons from other worlds, or were forced to flee in advance of legions of walking corpses that relentlessly tried to kill them and dogged their steps all the way to safety.
The humans who congregate today in centers like Stormwind and Theramore have survived when vast numbers of their people died. Only the former high elves have lost more of their kind. The fact that humanity manages to remain a force to be reckoned with despite the loss of almost all of its former northern domains in the Eastern Kingdoms, the deaths of uncounted numbers of their people and the usurpation of their inheritance is a testament to their origin as a seed race of the Titan's first arrival on Azeroth. Indeed, much like their dwarven cousins (for now humans and dwarves truly know they share a common origin, as do their gnomish relations), humans harbor a stony resolve in the face of adversity that could crush or corrupt another people.
Let us look at humanity's most recent travails.