The poem is only a few lines long, but it turns on its last two words. The speaker spends an afternoon irritated by a child crying somewhere at school. The word "vexing" suggests a low, nagging annoyance, the kind that wears you down without ever becoming a real crisis. The sound is something to be endured, and the speaker seems to treat it as someone else's problem.
Then the dash and the short line "I am he" change everything. The speaker realizes that the crying child is, in some sense, himself. We can't tell from the text whether he means a memory of his own childhood, a grief he has been holding in, or simply a recognition that his irritation hides his own distress. That openness is part of the poem's pull.
The mood begins in mild impatience and ends in quiet shock. The title, "Self-discovery," sounds almost clinical, which makes the moment feel more raw. The speaker finds himself in the last place he expected.