Now, closer to the millenium, we have 'techno', 'vanity rap', and 'happy music'. These are all relatively mindless forms of music. Techno's repetitive beats and samples are nothing more than hypnosis. Rap is now almost completely void of meaning, and has become the shallow and pointless thing gangsta rap was all along. Glorifying almost unattainable goals. The happy music is merely a sign of our anxiety, the world is changing, the majority of people have never been through a turn of the century (2000), let alone millenium (2001). No matter how level-headed people think they are, they can't escape the fear of change. The reason we now have these nonsensical music forms is because people need to turn to something that will dissolve their thoughts and worries in a bouncy, catchy tune.
Now people listen to cheerful music because there's a reason to need to be cheered up. The early nineties were merely a false start, represented by 'generation x''s whining.
This is not to say that there is actually an event or catastrophe that coincides with the next millenium, but culture has given us this embedded sense of fear and exuberance toward this event.