Being in the midst of trying to control mine, here is a basic explanation provided by my Endocrinologists with some help from Wikipedia.
It all comes down to the Renin–angiotensin system.
When the Kidneys think they don't have enough fluid volume to do their job, they send out signals that ultimately come back to themselves, causing them to retain salt and ditch potassium. Water, naturally, follows the salt and results in increased blood volume and subsequently, pressure. That's why many BP meds contains diuretics.
Too much salts mucks up all the complex feedback mechanisms.
Something that is apparently under diagnosed is a condition called Aldosteronism, where the adrenal glands make too much of a hormone called aldosterone, a primary messenger in this cycle. Aldosterone levels are not part of the standard blood workup done by your average family physician and diagnosing the condition requires a special, hours long process. It's is kind of a Zebra in diagnostic terms but is looking more and more like a horse.
So if you are afflicted with High BP and you're not having much luck controlling it, ask your doctor about aldosterone levels. High BP that is a result of this condition is more dangerous in the long term, but possibly curable by suppressing the aldosterone or removing one of the adrenal glands.