The first two lines present the poem's first of two distinct scenarios. Here, the speaker relates what happened when the speaker talked about anger that he or she was feeling toward a friend: talking about the anger made it go away. The straightforward simplicity of these lines suggests the ease with which the anger disappeared, and thus presents being open and honest as an extremely effective antidote to wrath.
Of course, in this scenario, the speaker's anger is directed towards a friend, someone with whom the speaker is familiar and, ostensibly, already likes. This raises the question of whether friendship is a prerequisite for open communication—whether the speaker was able to discuss his or her problem, and in doing so end it, only because the speaker was approaching a friend.
However, the use of "friend" here is likely a broader reflection of the stark division between the two ways of approaching not just anger, but other people more generally. Friendship, kinship, and common ground facilitate talking, which diminishes conflict—this in turn cements the positive relationship between people. In other words, the very act of the speaker revealing his or her anger can be seen as making the other person a friend.
Again, the shortness of this scenario reflects the simplicity of this idea: this part of the poem is effectively over by the end of line 2, formally reflecting the way in which the speaker's anger itself was quickly brought to an end. Because the anger disappears, there is no opportunity for it to grow into the poisonous tree that comes about in the second scenario. Given that the reader already knows from the title of the poem that there is a "poison tree" to come, there is a sense that these two lines present the innocent and unspoiled state of humankind, humanity's beginnings before the biblical "Fall of Man." This an idealized state of being, and the way the speaker implies humankind should be (but isn't).
The simplicity of these two lines is also sonically satisfying to the reader, with the perfect rhyme of "friend" and "end"—and the end stop at line 2—making it feel as though the poem could almost end right here. The meter, which shifts from trochees in line 1 to iambs in line 2, also has an easy, sing-song-like quality that further reflects its simplicity, with the final stress on "end" again suggesting how easy it would be for people to resolve—or "end"—their issues if only they spoke up about their feelings:
I was angry with my friend;
I told my wrath, my wrath did end.
Except, of course, this is a poem of experience—not idealism. The shortness of this first scenario, and the fact that it is followed by another fourteen lines, implies that the world, in actuality, is patently not like this first scenario. Accordingly, the reader knows that the poem must move away from its peaceful beginnings.