The main causes of stress for me (and I suspect most people) are I believe, worry and want. Worry, focusing on undesired events or outcomes, and want, focusing on desired events and outcomes. Both come from focusing on the future or past instead of living in the present.
Another way to think about the misplaced focus is to think of a bell curve, with past, present in the center and future. Focus should be like a bell curve with the majority of focus on the middle (now). Instead, with misplaced focus, people have an inverted bell curve, with more time spent on the past (rehashing) and future (worrying/wanting) instead of the present (living).
In this I believe that the Budda was spot on in identifying the root cause of human suffering as being desire. Desire can be as much the wish to avoid something (up to and including death) as it can be the want for something. When I look on the world as it in this moment and deal with it in this moment in the best way possible, I will work to free myself from desire and the suffering it causes me.