Tennyson once wrote a pretty great poem about "nature, red in tooth and claw." In other words, nature can be pretty brutal. In Yeats’s poem, that’s certainly the case. No matter who (or what) you are, if you have a body, you’re going to start decaying pretty quickly. The second we’re born we begin to die. Most importantly, this means that there’s absolutely no distinction between humankind and all the other creatures creeping, crawling, and flying around the planet. If you’re Yeats, the natural world is for the birds. Seriously. Humans have the ability to be more than just flesh…it just takes a little work.
The speaker’s urge to evade the laws of nature leads him into an escapist fantasy that values art more than the life than art depicts.
"Sailing to Byzantium" is a sublime vision of the ways that art can amplify the beauty of the natural world.