Everyone thinks that Okonkwo is so fierce and quick to anger because of his own inner fire, but it is very likely that his drive to prove his manliness and his emotional fire is because of the way that his farther was perceived in the tribe.  His father was considered "agbala" or weak and so Okonkwo is driven by the internal fear of being thought similar to his father.  Because of this he works incredibly hard and also appears to have developed a very short fuse.  He's been ruling everyone with an iron fist just to be sure that his father's memory will not be tied to him.