"Gay your life must be"

As a child I always had an issue with that line from the traditional 'Kookaburra Sits in the Old Gum Tree' but not because of any reference to sexuality. I did not erupt into childish fits of laughter but was rather perplexed at the meaning of the line. In particular I struggled with the word "MUST". "Gay your life MUST be" sounded to me like a rather threatening demand. Why must the kookaburra be happy and exuberant all the time? What if he was having a rough day? What if he was suffering from serious depression? Is he meant to mask those emotions and put on a jolly exterior for the amusement of others? Why is he unable to express his true self and forced to play to some ridiculous stereotype? Who came up with this rule in the first place? I'd like to see how they would cope with such an outrageous demand! These poor kookaburras are being allowed no expression of self which can be quite damaging......oh no hang on a minute I see now. "Gay your life must be" as in "you must be having such a wonderful time in your life as you seem to be laughing all the time" not "you must have a happy life or there will be terrible consequences". Still, it could be all fake laughter.