http://www.webmd.com/hypertension-high-blood-pressure/guide/blood-pressure-causes

The exact causes of high blood pressure are not known, but several factors and conditions may play a role in its development, including:

Smoking
Being overweight or obese
Lack of physical activity
Too much salt in the diet
Too much alcohol consumption (more than 1 to 2 drinks per day)
Stress
Older age
Genetics
Family history of high blood pressure
Chronic kidney disease
Adrenal and thyroid disorders

Of the approximately 95% of reported cases in the U.S., the underlying cause cannot be determined. This type is called essential hypertension.

Though essential hypertension remains somewhat mysterious, it has been linked to certain risk factors. High blood pressure tends to run in families and is more likely to affect men than women. Age and race also play a role. In the United States, blacks are twice as likely as whites to have high blood pressure, although the gap begins to narrow around age 44. After age 65, black women have the highest incidence of high blood pressure.

Essential hypertension is also greatly influenced by diet and lifestyle. The link between salt and high blood pressure is especially compelling. People living on the northern islands of Japan eat more salt per capita than anyone else in the world and have the highest incidence of essential hypertension. By contrast, people who add no salt to their food show virtually no traces of essential hypertension.

The majority of all people with high blood pressure are “salt sensitive,” meaning that anything more than the minimal bodily need for salt is too much for them and increases their blood pressure. Other factors that can raise the risk of having essential hypertension include obesity; diabetes; stress; insufficient intake of potassium, calcium, and magnesium; lack of physical activity; and chronic alcohol consumption.

(The remaining 5% of cases are referred to as secondary hypertension. These cases are typically related to identifiable medical conditions or the effects of certain medications and treatments. To the degree that the secondary medical problem is reversible or the offending medications may be safely discontinued, true secondary hypertension is curable.

For all practical purposes, when people and doctors speak of hypertension they generally mean essential hypertension.)