Hmmmm.... I think people are such creatures that get used to things and after that when nothing actually surprises them anymore they start looking for something more thrilling...same, i guess, happened with me.
I've started from Britney Spears and now I'm sitting in a J-music world. I grew up with music that my dad used to play (the beatless, queen, ozzy....) so it changed my taste too after I started to realize that pop is too sweet for me. I went straight to the gothic rock.. What I'm trying to say is that I lisved with English music and just by little accident I found the j-music world. But then again, Ayu was first and when I listened to GACKT I was like "Who could listen to that?!
" yeah...
the point is music taste changes with the person and their life, at least this is how I see that.
Right now I listen to mostly japanese, but I do still have my old western favorites from years ago. The main thing I usually look in the music is the richness of the melody, and the weird, unexpected sounds and solutions melody/vocal-wise, or just the melodic sound in the song which makes it so easy to listen and so catchy. I like finding new sounds while listening to songs over and over again, they simply don't get you borred. These are the only qualities that i actually find more in japanese than in western music.
To add to this, even my dad admited that at some point japanese music is different and fresh for the ears of those who never really heard any j-music before, I'm refering to rock in this case, since me and my dad we both are rock music lovers.