The answer is best explained with an analogy. You're holding a hose, you turn on the water, and it shoots through the hose and out the nozzle. The water is moving because it's under pressure. Similarly, when your heart beats it creates pressure that enables your blood to "water" your body with the nutrients that are in your blood. Three things affect the amount of pressure that pushes blood through your body: how forcefully the heart pumps, how much blood there is, and how narrow the smallest blood vessels are. That last one may need some explanation. When you're watering plants, and you want the stream of water to go farther, what do you do? You make the nozzle smaller. Narrowing the opening through which the water flows increases the pressure, and the water then is able to reach the distant flowerbed.
What affects how hard the heart pumps? Several body hormones, particularly epinephrine and norepinephrine, "whip" the heart to beat faster and harder. These hormones are made by your adrenal glands (located just on top of your kidneys). Exercising and fear cause increases in these two hormones, which is good: your heart needs to work harder when you exercise, or when you may need to run away from something frightening. However, chronic anxiety and stress also increase these hormones, and that's not good. During a stressful day, no good purpose is served by your heart beating faster and harder, and by your blood pressure going up. Some uncommon diseases — not just chronic anxiety and stress — also can raise levels of these hormones.
What affects how much blood you have? Blood is part water. Therefore, the amount of fluid you drink and the amount you lose through sweating and urination affect how much fluid there is in your blood. Blood also is part salt. Salty foods and salt added to your foods can increase the fluid in your blood. That's why people with high blood pressure are urged to go light on salt, and often given diuretic drugs to help their kidneys eliminate excess fluid and salt. There also are some uncommon diseases that can increase the amount of fluid in your blood. What affects how narrow your small blood vessels (arterioles) are? These blood vessels have circles of muscle in their walls that can clamp down and narrow the vessels. The same hormone — norepinephrine — that whips your heart to beat faster and harder also causes the blood vessels to clamp down, raising blood pressure.
In other words, high blood pressure — like most diseases — is influenced by our genes and also by our lifestyle. Currently, we can do a lot more about the latter. The easiest would be to get moving more.