Blame your parents. Well don't blame them, i was just kidding. But the problem runs in families. Diet especially high salt intake can lead to hypertension in some patients. As people get older they are much more likely to get high blood pressure. Some rare causes are related to things like tumors in the adrenal glands or kidney problems.
Many mechanisms. Most causes may actually be unknown, but run in families. Diseases of the kidneys and hormonal imbalances can be he cause. More commonly, being overweight, excess dietary sodium, medications and alcohol can be the culprit. Seek medical attention since lifestyle and, if needed, pharmacologic therapies are very successful.