I actually don't understand your point at all.

Before Keats found out about what rainbows "really are" he experienced wonder while looking at them. After, he didn't.

What else is the man supposed to do? He's got to try to investigate his experience, right? Where did he go wrong?

You are reducing his cognitive processes to those of a bumbling fool. They're complex, you just don't understand them. It doesn't seem like you're making enough of an effort.