Many factors. Family history that means genetic reason, overweight/obese, high sodium diet, sedative life style, etc.
Many things. In most cases it is idipathic (you just have it, we don't know why). It can be caused or made worse by stress or high salt intake. Certain drugs such as pain medications can elevate the blood pressure. There are rare specific causes such as tumors of the adrenal glands or narrowing of the aorta, . It runs in families as well so you might blame your parents.