Some people like to say that all art is ultimately about two things: love and death. Shakespeare, though, never one to be complacent, noticed that it’s silly to reduce art to two things that don’t take into account the medium through which they’re presented. In other words, a big theme for Shakespeare is artistic self-reference – the Bard is never going to let you forget that what you’re reading or seeing performed isn’t the same thing as real life. You can see this all over the place in his major plays. Check out the play-within-a-play in Hamlet, the epilogue of As You Like It, or the entire The Tempest for a couple of examples.
Shakespeare completely refuses to let you feel comfortable in a world of fiction. He’s going to remind you, over and over again, that what happens on stage, or, in this case, in a poem, is an artistically created world, and so works differently than our world. Still, though, this fake world is a really important part of our world, and he knows it. In other words, the world of fiction isn’t the same thing as our world, but it’s really important to understanding how our world works. And Sonnet 18, as you might have noticed, is basically the epitome of this kind of thinking. Shakespeare basically says, "this isn’t a love letter, it’s a poem, and you’re going to like it. In fact you’re reading it right now. Checkmate."