There is no consensus as to why kidney stones form. However, some of the reasons can be:
1) Heredity: Some people are more susceptible to forming kidney stones, and heredity may play a role. The majority of kidney stones are made of calcium, and hypercalciuria (high levels of calcium in the urine) is one risk factor.
2) Geographical location: There may be a geographic predisposition to forming kidney stones, so where a person lives may make it more likely for them to form kidney stones.
3) Diet: Diet may or may not be an issue. If a person is susceptible to forming stones, then foods high in animal proteins and salt may increase the risk; however, if a person isn't susceptible to forming stones, diet probably will not change that risk.
4) Medications: People taking diuretics (or "water pills") and those who consume excess calcium-containing antacids can increase the amount of calcium in their urine and potentially increase their risk of forming stones.
5) Underlying illnesses: Some chronic illnesses are associated with kidney stone formation, including cystic fibrosis, renal tubular acidosis, and inflammatory bowel disease.