Here's why:
* Regret: The narrator realizes his own pride and ambition were the driving forces behind Doodle's death. He pushed his brother too hard, neglecting Doodle's limitations and physical frailties. He feels a deep remorse for pushing Doodle beyond his capabilities.
* Guilt: The narrator acknowledges his responsibility in Doodle's death. He knows he could have been more patient and understanding, instead of demanding perfection from his brother. This guilt weighs heavily on him.
* Loss: The narrator mourns the loss of his brother, both physically and emotionally. He loses his companion, someone he loved deeply, and the bond they had.
The narrator's final words, "For a long time, I thought my brother's death had been a blessing, but now I know that it was only a part of my education, a part of my coming to know that the world was not a fit place for Doodle and that he was better off dead," reveal his profound sadness and the heavy burden of guilt he carries. He understands that he was partly responsible for Doodle's death and acknowledges the unfairness of the world that couldn't accommodate his brother's fragility.
In short, the narrator ends the story with a heavy heart, filled with regret, guilt, and a deep sense of loss for his beloved brother.