Q: (there's some explanation about the word "la di da" but I don't think Bill is referencing to that la di da)  ---The tune (or the sound, music, I don't know what you actually call them) is a joyful expression. The la de da part sounds happy. Combine this with the meaningless of the lyrics, this means happiness without reason.
A: i think the problem is you are blatantly ignoring all the meaning