As humans, we seem incapable of holding on to both hope and despair at once. That is why, when we are despairing and are offered faint hope, it turns so quickly to certainty. If you're dying of cancer, but the doctor offers you an experimental treatment that may be effective in 1% of cases, you'll believe yourself to be in that 1%.
Mokalus of Borg
PS - This is also why the same people keep buying lottery tickets.
PPS - Maybe we're not supposed to be pessimists.