The speaker implies early in the poem that he is somehow responsible for his son's death. He says he had "too much hope" for his son, and that this was his "sin" (2). The language of punishment is present elsewhere in the poem as well. A payment is "exacted" from the speaker, and the concluding lines of the poem reflect the speaker's wish not to make the same mistake again (i.e., not liking something too much).
Line 2: The speaker says his "sin" was having "too much hope" for his son. It is a "sin" because the speaker failed to realize that his son's life could end at any time.
Line 4: The son's fate punishes the speaker. His death is "exacted" from the speaker. "Exacted" is a metaphor to describe how God or Heaven takes the son back as if they were taking back some kind of payment.
Lines 11-12: The speaker, for his own sake, wishes to never like "too much" the things he loves. He implies that this leads to punishments of the kind he has just suffered.