The whole reason that the speaker is talking to Margaret is that she is crying over the dead leaves she finds in the forest of Goldengrove. The speaker then starts musing about what the real reason is that she's so sad. It's about the leaves, but it's also about death and mortality more generally, he says. And, not to bum you out, but that's something that everyone has in common.
Line 1: "Grieving" rhymes with "unleaving" in the first line—it's an internal rhyme. Having that internal rhyme right off the bat like this gives the impression that the poem is going to be sort of sing-songy, like a poem for children. And since we know from the dedication that it's addressed "to a young child," this makes sense. Of course, the topic of the poem (death and mortality and sorrow) might not be the most kid-friendly…
Line 5: Obviously it's not just her "heart" that will "grow older." The speaker uses synecdoche when he refers to Margaret's "heart" standing in for all of her.
Line 7: "By and by" rhymes with "sigh"—more internal rhyme! And we have another example of alliteration in this line—the repeated "s" of "spare a sigh." Both the alliteration and the internal rhyme help draw attention to the "sigh" at the end of this line. Why do you think Hopkins would want to draw attention to this line, in particular? This is the point in the poem where he says that people stop "grieving" (1) or "sigh[ing]" (7) over dead leaves when they get older. Instead, they cry over their own eventual deaths.
Line 9: The repeated "w" sounds in this line are yet another example of alliteration.
Line 11: When the speaker describes the sources of sorrow as "sorrow's springs," he's using a metaphor. A "spring" is the source of a river (like a mountain spring). It makes sense to use this river metaphor to describe the sources of sorrow since, after all, sorrow makes us cry rivers and floods of tears. "Sorrow's springs" is also—you guessed it—an alliteration. In fact, the repeated "s" sound is a special kind of alliteration called sibilance.
Lines 14-15: This poem uses a special kind of end rhyme pretty often, and we see it here at the end. It's a feminine rhyme, or a rhyme of two or more syllables. Check it out: "born for" rhymes with "mourn for." Feminine rhymes are often used in children's verses or comic poems (like in poems by Shel Silverstein or Ogden Nash), so the closing couplet of this poem adds to the sense that it's a poem for children—in spite of the very adult topic of the poem.