This etude is semi-ironically written after a happy day. But instead turned out to be more of a reflection on thinking and feeling in general, instead of just plain happiness. Which are really all invention of the mind anyways when you think about it. The repeated trills (quick notes) could be interpreted as the start of new thoughts, with the right hand repetition being the constant buzz your mind makes. And of course fitting to the subject the piece finishes in an unfinished sort of way.