In a relationship between friends, there can be disagreement and even anger, certainly, no matter how good the friendship is.  When we value a friendship, we express our disagreement or anger, we talk it out with the friend, and then it is over and done with, we hope. This is in sharp contrast to what the narrator does with his foe, isn't it?  He does not express his anger; he keeps it to himself and nurses it, feeding it, so it grows larger and stronger, ultimately strong enough to poison his foe and kill him. The idea is that when we vent our negative feelings, we are able to be done with them. Those negative feelings we do not express are far more difficult to get over because, the narrator believes, they fester, like an untreated wound. There is disagreement amongst psychologists about this, with some thinking that "getting it all out" is mentally healthy, and others focusing more on changing behavior rather than cathartic release.