The speaker has several reasons for preferring the birch trees be bent by boys and not by ice storms. First, the ice is harsher and relentless, leaving the birch trees permanently bent to the earth and unable to stand upright again. As he puts it, ending with a wonderful image of girls drying their hair in the sun:
And they seem not to break; though once they are bowed
So low for long, they never right themselves:
You may see their trunks arching in the woods
Years afterwards, trailing their leaves on the ground
Like girls on hands and knees that throw their hair
Before them over their heads to dry in the sun.
The speaker would prefer the trees to be able to bounce back and become upright again.
Second, boys are not only gentler when they bend the branches, they experience joy in the birch trees, and they learn how to handle nature—and life—in a humane way. They learn how to interact with the tree to that it doesn't bend all the way down:
He learned all there was
To learn about not launching...
(The entire section contains 2 answers and 673 words.)