Guilt

Because this story is framed as a retrospective, there is a lot of room for the narrator’s guilt to come through. The narrator flashes back to this time in his life with a wistful, guilt-ridden tone; it is clear he blames himself for Doodle’s death, even though Doodle was extremely unhealthy to begin with and other factors came into play as well. 

Recognizing Limits

It can often be tempting to push ourselves and the people we love past their limits in the hopes of achieving a goal, just like what happened with Doodle and the narrator. Sometimes this produces great results; after all, Doodle did learn to walk after working extremely hard. The author continued to push doodle past his breaking point.