"Consider an experiment by researchers at Maastricht University in the Netherlands who gave subjects a series of 20 electric shocks. Some subjects knew they would receive an intense shock on every trial. Others knew they would receive 17 mild shocks and 3 intense shocks, but they didn't know on which of the 20 trials the intense shocks would come. The results showed that subjects who thought there was a small chance of receiving an intense shock were more afraid — they sweated more profusely, their hearts beat faster — than subjects who knew for sure that they'd receive an intense shock.From The Situationist
That's because people feel worse when something bad might occur than when something bad will occur. Most of us aren't losing sleep and sucking down Marlboros because the Dow is going to fall another thousand points, but because we don't know whether it will fall or not — and human beings find uncertainty more painful than the things they're uncertain about.
For the record, this typically does not apply to me. I am fine with uncertainty, and deal with things as they come. But I have known for some time that this is a rare skill.