Due to Zheyli, God of Emotions being such an influencing deity in this world, strong emotions can have unexpected side-effects. Usually not enough to be noticed, but barbarians are people who have trained this effect to an art.

Not only anger, a barbarian's "Rage" is usually whatever strong emotion a given barbarian is most in tune with (its usually anger, but you can get creative about what your barbarians rage state is like :) ).   There are many sorta 'classic' version of barbarians who worship Polemos, Goddess of Conflict, and put themselves in positions of constant conflict to get stronger! This is also applicable for Warriors and others.