Method: contrast and foil.
This little poem is about her accompanying her father on a vacation in the south of France, but she returned early due to her father's health, and his father died soon after. A detail in life is left forever and is infinitely magnified in emotions. The balance between the old man (facts, appearance) and the child (feelings, heart) gradually tilts from the beginning to the end. What is left to the author and readers in the end is not death and aging, but the eager child. The fact is not true in the memory, because the poet sees that the old man is just a disguise; through this disguise, she sees an eager child. How can we not admit that there is something that will never die?
The most critical thing about the whole poem is the disguise in the last line, and the middle sentence "What makes me sad" becomes a sudden change in the drama.