Hypertension is caused by a variety of mechanisms. Body water is one, but assuming decent access to water when thirsty, this is controlled not by where you live and sweat (barring real dehydration), but by an intricate cascade of hormones from the kidneys, adrenals, and elsewhere. Other factors include how forcefully the heart beats (like how much pressure is delivered to the faucet), how much the blood vessels are contracted (like narrowings in the hose) or inflexible, diet (salt, potassium, etc. affect hormone output), exercise, and adrenalin and similar chemicals some of which are increased by stress. Obesity and alcohol can also bring about hypertension, in these cases often reversible. White-coat hypertension may be a separate case but is often a harbinger of constant hypertension down the road.
The biggest problem in managing hypertension is that patients refuse or stop taking their medicine. They come in after a couple of months of treatment and their pressure -- finally -- is normal. They sometimes ask why they need to keep taking the medicine and I explain that this is why their pressure is now normal. But with a lack of symptoms they fail to comply. 5 years later: stroke, heart failure, kidney failure etc. start to creep into this population. Not most, but more than necessary.
I'm pretty lean in my approach to management, but I treat BP and cholesterol vigorously and according to the best current evidence. It works. Don't usually need fancy or expensive drugs: hydrochlorothiazide 12.5mg is an old standby, cheaper than dirt, and probably still the single most effective agent we have. Only need the big guns when the old standars don't work.
BP will fluctuate quite a bit with stress or loss of stress; it will often stay low for months after stopping BP meds (though some meds are dangerous to stop cold-turkey), and it will just float around quite a bit for unknown reasons. But if you have properly diagnosed hypertension, it will almost alway revert to its elevated levels without treatment. Don't want to get too complacent when it's down for a month or two and if meds are stopped, they often need to be restarted 6 months later.