Why does hypertension occur?

Genes and time. The arteries (vessels that transport blood away from heart) are very elegant pipes lined with muscle and smooth tissue to allow blood to travel quickly where its neeeded and to stretch or pulse based on body needs. As we get older, cholesterol deposits make them stiffer and less flexible increasing the pressure. High dietary sodium, kidney disease, obesity can also increase the volume of blood.
Many reasons. Overweight, high sodium intake, genetic inherence, stress/anxiety as well as lots of pathological situations.