The song's lyrics are based on Japanese onomatopoeia, the colourful Japanese phrases which express something like an emotion by copying its "sound" -- even if it doesn't technically have one. (What's the sound of a painful condition? Zuki zuki. A glittering spakle? Why, pika pika of course!). You could see the song as an ultra-emotional way of learning a language.
The video shows a sequence of people I happened to meet in New York in April. I asked them to look sad, but the atmosphere of the song is perhaps best described as sensual, full of mono no aware (the "sigh-ness" of things) and natsukashii, nostalgia, or the longing for that which has never been. Nostalgia for an absent person, perhaps, or one you haven't met yet.
Speaking of love and couples, the article you helped me with last week, The Kinsey of Clicking, is now up on Wired News. Thanks to everyone who participated!