My interpretation is that Billy Joel is saying that the difference between moods weather very happy or really down, like the speaker in this song, is only a minor variation. He is saying that your always yourself but your mood is what varies; so, if the speaker doesn't seem like himself it not a complete change but just a minor variation.
My interpretation is that Billy Joel is saying that the difference between moods weather very happy or really down, like the speaker in this song, is only a minor variation. He is saying that your always yourself but your mood is what varies; so, if the speaker doesn't seem like himself it not a complete change but just a minor variation.