If you look at the canon of Scots/Irish/English traditional song, the majority of them are sad songs. It certainly appears that personal loss and hardship (being the norm in past centuries) have been the inspiration for more great songwriting than have happy circumstances.
"No whining in song" would be a maxim that trad lovers apply to contemporary writing, somewhat unfairly, I'd say, as there is lots of lamenting (=whining) in the old songs. Why is it that that's OK with us, but new writers get slagged for it?
It's been said (by whom??) that all real artists must suffer to produce their art. I don't know whether that's true, but I do know that I derive more pleasure from listening to a heartfelt lament than a jolly ditty. It just gets me more emotionally involved -- which is what I ask from music.