Only 5 -10% of cases of hypertension have causes (e.g. narrowing of the arteries) which can be clearly identified. Put another way, of those patients who have been identified as having abnormally high blood pressure, physicians will not be able to pinpoint a specific cause in more than 90% of cases.
“Essential hypertension (also called primary hypertension or idiopathic hypertension) is the form of hypertension that by definition has no identifiable cause. It is the most common type of hypertension, affecting 95% of hypertensive patients, it tends to be familial and is likely to be the consequence of an interaction between environmental and genetic factors.”
“More than 90% of cases of hypertension do not have a clear cause. Hypertension clusters in families and results from a complex interaction of genetic and environmental factors.” American College of Physicians, Annals of Internal Medicine.
The complex pathways which regulate blood pressure in humans have been the subject of intense research for many decades. Although many mechanisms have been discovered and described (see Wikipedia) there is currently no overall and complete model. It's known for example that salt intake has a large effect in raising blood pressure, but the exact mechanism by which it does so is unknown. Pressure-altering effects of the sympathetic nervous system, and of exercise, are also lacking detail.