Years ago diet, ph created by diet, and or magnesium levels often were the culprit, but since the 80's commercial diets have corrected these issues, yet we still see the problem.  In older cats we do see a significant amount caused by bacteria, some by stones, tumors, or other physical causes.  In younger cats 50% show no cause in spite of extensive testing.  This condition is often referred to as idiopathic cystitis. Idiopathic meaning we don't know the cause.  We are currently thinking it may be a stress related condition.  We urge you to learn more about this condition in the pet library from veterinary partner.  We have included a direct link to the idiopathic cystitis topic in the pet library.