In this poem, the persona fed his anger by burying it inside of himself. In the first stanza, he contrasts this approach with his better behavior toward his "friend." When the persona becomes angry at his friend, he discusses it with him and, as a result, gets over it. However, when he grows angry at his foe, or enemy, he doesn't tell him about it.
In addition to not expressing his anger to his enemy, the persona feeds his anger in other ways: he dwells on his anger all the time, never letting it go and in fact, grows more fearful of his foe. As he puts it, he "water'd" his growing fear "night and morning" with his "tears." He also feeds his anger by pretending to be nice to his enemy, all the while inwardly seething.
The poem expresses how poisonous it is to nurse a grievance and how destructive that can be.