Have you ever noticed how profoundly sad a song about happiness can be--- "Blue Skies, Smiling at me. Nothing but Blue Skies, do i see."?
Maybe Irving Berlin wasn't writing about being happy at all. Was he constructing that veneer of sunshine behind which hid a deep, inescapable melancholy? Was it the great depression when he wrote this song- when poor people could escape to dreams of emerald cities and wizards of oz?