It's called Happy Hardcore (and sometimes Fun Core and Happy Gabba) because it's an offshoot from the
Hardcore/Breakbeat Techno scene, as much as
Gabber is. The Hardcore basics are all there: fast (160-200 BPM) beats,
distorted/compressed kick drums and claps, furious crescendo snares, and the sliding edgy synths. It's fused with
Eurocheese downbeat basslines and uplifting (sentimental? cheesy!) lyrics. And yes, all the melodies are in a
major key.
I've been listening to Hardcore and Gabber since 1994, when Rotterdam and Rob Gee ruled the scene. At that time, it was apparent the style would progress. How could the scene not evolve from the I-want-to-fuck-you-til-you-bleed, fuck-you-and-your-mother, are-you-prepared-to-die type songs? Like the way the pop charts shifted from grunge to bubblegum pop in the mid-90's, Happy Hardcore was a reaction to what had become a stasis.
Some artists have gone with the change (or sold out, depending on your perspective). Charly Lownoise and Mental Theo, who have often appeared on Thunderdome albums, released songs like Your Smile and Next to Me. Some artists like Paul Elstak manage to keep their tracks rooted in Hardcore. Others, like Bang! (responsible for the Happy Hardcore anthem Shooting Star) barely resemble their predecessors.
To me, Happy Hardcore is another sign of electronic and dance music's maturity. To have a sub-genre that is so silly gives more credence to its parent.