Humankind has relatively diverse martial tradition, but throughout them all there is a common thread of bravery and recklessness. When humans go into battle, there is little that inspires them more then the sight of their commanders leading the way. This can of course be said for many other races, but when combined with human recklessness, the practice among their kind is alarmingly common.
It deserves mention that not all humans live up to the standard set by there greatest (and probably dead) heroes. But even those shrewd souls who eschew physical conflict can occasionally be stirred into action by greed or vengeance.