I was angry with my friend; I told my wrath, my wrath did end. (1-2)
The poem's first two lines show how anger can be literally contained or curbed: by talking about it! They enact this theme formally as well. In line 2, the words "my wrath, my wrath" appear sandwiched (this figure is called chiasmus) between "I told" and "did end" in such a way that suggests talking is a way of containing anger and making it go away.
And I watered it in fears, Night and morning with my tears; And I sunned it with smiles, And with soft deceitful wiles. (5-8)
The repetition of "and" three times in this stanza illustrates how closely linked the development of anger and a lack of communication are. It is as if the poem is saying "I didn't speak about my anger and then x happened and then y happened and then z happened."
And it grew both day and night, Till it bore an apple bright. And my foe beheld it shine. And he knew that it was mine (9-12)
Neither the speaker nor the "foe" speak to each other. The communication here is entirely non-verbal, and it leads to tragic consequences (for the enemy, at least). The speaker and his foe employ other senses and physical gestures (in this stanza it's sight; in the previous stanza it's tears, fears, and smiles), rather than communicating directly, and death results. The poem champions speaking and listening over seeing.