You've probably heard the cheesy saying: "Life isn't about how many breaths you take, it's about how many moments take your breath away." (What? We warned you it was cheesy.) Still, there is some truth to it; we measure our lives by the moments we are struck stupid by something so beautiful or amazing that we can only stand and drool. In "On First Looking into Chapman's Homer," that moment for Keats is "hearing" (more likely, reading) Chapman's translation. Something about the language and the imagery just floors him, and he knows he will never be the same. (Thankfully, he was able to churn out a poem with a much lower cheese factor.)
Hang on a second, Keats ol' buddy. Awe and amazement are completely personal and subjective—what totally floors one person might not affect anyone else as forcefully.
Keats feels that it is the artist's job to take those moments of awe and recreate them for others in their work. Now try putting that on your resume.