What causes the speaker’s anger to grow in stanza 2?

This poem by William Blake concerns the nature of anger and resentment, and the fact that "wrath" untold is likely to take root and grow. In the first stanza, the speaker describes how, when "told" to a friend, anger swiftly dissipates. The rest of the poem, however, contrasts this situation with instances where, when angry with "a foe," we do not express what is bothering us and, instead, allow the feeling to grow.
In the second stanza, the "wrath" is imagined as a literal seed, which is watered with "fears" and "tears." This suggests that the speaker allowed his paranoia and distress about whatever was bothering him to build up into something greater than it had been before. Eventually, he came to feel almost fond, or possessive, of his secret bitterness, as suggested by the image of its being "sunned" with "smiles" and "nurtured" with "wiles," or cunning. The speaker has reached a point where he will go to great lengths to prevent others from destroying or minimizing his bitterness.