Too much of a good thing can kill you.
Like most of the younger generation of Romantic poets (including Lord Byron and Percy Bysshe Shelley), John Keats liked to write poems celebrating youth, sex, and beauty. "La Belle Dame Sans Merci" is no exception – except that we're not exactly wishing we were in the knight's shoes at the end of the poem. Sure, he gets the girl, but then she dumps him and leaves him out in the cold (literally). Keats seems to be pointing out to his readers, "sure, guys, sex and beauty are great, but if you obsess about them to the exclusion of everything else, you're going to die. Alone." This is advice that rock-star poets like Lord Byron could probably have used.