Allusions to Greek Mythology
For someone who didn't have a fancy-schmancy formal education and was never taught ancient Greek like many of his contemporaries, John Keats sure did love throwing allusions to ancient mythology and literature into his poems.
Some critics say that this was because he was insecure about his lack of formal education—that Keats was trying to show the world that he knew just as much about Greek literature as anyone who went to a fancy boarding school and university. But perhaps more importantly, ancient Greek writers and philosophers had a lot in common with Keats's philosophies about the nature of pleasure and pain, life and death, so it makes sense that he would include these classical allusions in his poetry.