"Spring and Fall" opens with a little girl, Margaret, "grieving." What's she so sad about? The speaker offers one possibility at the opening of the poem—she's sad that the leaves are falling off the trees. But by the end of the poem, he says that she's sad about something different. It probably has something to do with the ever-present specter of Death that the speaker has pointed out. Thanks for that!
Misery loves company, gang. The speaker of this poem shares Margaret's grief, although, as an adult, he's better able to explain the source of his sorrow, both to himself and to his young friend.
It's not so bad. Although the poem is about a young child learning about mortality, the tone of the poem is remarkably hopeful.