Dude that is exactly what I asked too! The drs there sucked. They said no one knows what causes them, most people believe it has to do with your diet, they couldn't tell me what though. I have not drank any soda or anything with carbonation for 1 year now. I drink at least one 1.5L bottle of water a day. Maybe one glass of unsweetened iced tea a day. I consider myself to be pretty healty...
The only thing I can think of is lately during the holidays I have been eating A LOT of chocolate
Through my own research I have found that calcium build up is a common cause. That and water fluoridation. I did find out that there is fluoride and calcium in Fiji water...maybe that has something to do with it idk.
All I know is I am going to switch water and try to drink some cranberry juice occasionally. I thought I was too young to get a kidney stone...but I guess not. <_<