Margaret, are you grievingOver Goldengrove unleaving? (1-2)
The falling of the leaves in the forest of Goldengrove seems to represent the fading of human life, as well. Margaret, as a little kid, is able to "grieve" for the trees themselves, but the speaker thinks that there's something more to it—she's becoming conscious of human mortality.
And yet you will weep and know why. (9)
In the future, Margaret will still "weep" in the autumn at the falling of the leaves, but it won't be in innocent grief over the trees—she will "know" that all humans are going to die one day, just as the leaves wither and die each year. So, she has that to look forward to, along with college applications and SAT prep.
Sorrow's springs are the same. (11)
The speaker says that the "springs," or sources of sadness, are the same for adults as they are for children, although adults might be able to express their feelings more effectively or more accurately. This might suggest that we should take the grief of children more seriously and treat them with more respect. So next time you see a kid crying over something that you think is trivial, give him or her a break!