I wonder that he didn't stop to wonder how amazing it was that light reflecting through a bunch of water droplets could create such a beautiful image in your mind.

That's roughly what I think when I see a rainbow. Same with things like sun dogs, or a gorgeous blood-orange sunset.

Just because I understand what is happening doesn't mean I find them any less beautiful. And besides, suppose I'm watching such a scene with someone who doesn't understand what they are seeing? I get to explain how such a thing can be, and in some cases how easy they are to reproduce.

I dunno, I find that pretty awesome myself. I honestly don't think Keats could have actually understood how rainbows formed and still feel cheated.