Q: And now for the main part, the la de da part  And it goes La de da de da de da de day oh La de da de where did all the good times go? ( La de da de la da dee d
 um dom L
 a da de day When will you come back home?  ---I believe that this part (and all the other la de da part) is referencing the idea of genuine happiness,
A: the lines immediately before this were basically begging to write a song. we have already essentially looked in the mirror and stated the wish 3 times, pretty much there is nowhere else to go but to actually do it. "La de da" etc. would be a meta characterization of a song being written