There isn't always a reason. The vast majority of people who have high blood pressure
, have what is known as 'essential hypertension' this basically means that the cause in unknown. It can sometimes happen in fit and healthy young people. Do you have a family history? or any other health problems?
Secondary causes for hypertension needs to be ruled out first. Secondary causes are problems to do with hormones and kidney's etc which can have an effect on BP. Single BP readings should never be considered diagnostic. For a more accurate analysis, BP should be measured over a minimum of 24 hours. 'White coat syndrome' is very common, which is why hypertension diagnosis is never made on one single reading.
Long term stress
is not linked to hypertension as some wrongly assume. Try and stay as close to your ideal weight as possible, eat healthily, reduce salt intake (no more than 6g a day), and exercise.