Why do authors vary sentence structure in their writing?

Read this adapted excerpt from a famous poem by john keats. this poem is about a beautiful vase with intricate shapes and patterns. you, silent form, do tease us out of thought as does etemity: cold pastorall when old age shall this generation waste, you shall remain in the middle of other woe than ours, a friend to man, to whom you say "beauty is truth, truth beauty that is all you know on earth, and all you need to know what is the poet saying when he writes that "old age shall this generation waste"? the poet is expressing his dread of growing older the poet is mourning the changes he sees in his "generation." the poet is saying that beauty is illusionary and short-lived. the poet is saying that people don't live long; life is brief