Unfortunately in most cases the cause of high blood pressure is unknown because it could be caused by multiple factors that are difficult to determine for sure. However, there are some known causes of hypertension like kidney problems, diabetes, hormonal imbalance, abnormalities of the aorta. Also some medications including birth control pills can cause high blood pressure. Also, if you smoke or drink a lot, or if it runs in your family, you could be at an increased risk for hypertension.
If left untreated, over time high BP can cause stress on some organs, like the kidneys and the heart, and can lead to heart attack and stroke, although this is most common in older people.
Has your doctor run tests yet to try to determine what's causing your high blood pressure?