The Question and Answer section for The Book of the Duchess and Other Poems is a great resource to ask questions, find answers, and discuss the novel.
The nymph's reply might be called realistic. She basically says that everything the shepherd says would be lovely if only people didn't grow old and change. The fact that the first line starts with "If" shows that she thinks that...
Is this specifically from "The book of the Duchess"?
... people become too concerned over meaningless things
.... there are rewards for putting your all into something and doing the best you can
.... anarchy, a government which does not govern is not a government