In the poem "A Poison Tree" why did the persona's wrath end when he told his friend about it?

In "The Poison Tree," the point of the poem is that the anger we hold onto grows into something ugly and poisonous that hurts other people. Blake contrasts this to anger we get over. One good way to overcome anger is to talk about it with the person who angered you. If you can trust another person enough to confide in him and her, you can clear up misunderstandings and give the other person the opportunity to tell his side of the story and explain what happened. It also offers the other person the chance to apologize, because often people don't even know they have offended. With this kind of communication, forgiveness and reconciliation can occur. Then you can move on with your life. In contrast, as the poem shows, if you dwell on your wrath without trying to resolve it, it becomes bigger and bigger and more and more poisonous--Blake imagines it as a poisonous apple--and becomes  especially poisonous if you pretend to the person who offended you that nothing is wrong.