Human beings are not creatures of logic and reason. We are ruled by our emotions—our fears, doubts, hopes, and dreams. That is why, even when presented with overwhelming scientific evidence of the veracity of something, people will often choose to deny that it could be so. We see this regularly with global warming deniers (ignoring the evidence and the experts) as well as with young-Earth creationists (ignoring scientific evidence about the age of the Earth).
NewScientist issued a special report a couple of days ago titled Seven reasons why people hate reason, and it's an interesting analysis of the underlying explanations for people's aversion to logic and reason. One of the biggest that is pointed out (by sociologist David Miller and linguist Noam Chomsky) is that governments and corporations often co-opt and pervert scientific results in order to further their agendas.
At the same time, Michael Shermer points out in an article for Scientific American how our brains are wired to apply anecdotal evidence to decision making, but not scientific evidence. This, Michael says, is the reason why people incorrectly associate vaccinations with autism in spite of scientific research repeatedly finding no link. In fact, people continue to put their children at risk of known diseases by avoiding vaccinations, even though thimerosal (the chemical they incorrectly believe causes autism) has not been used in vaccines since 1999.
Michael also highlights dubious claims about the natural health benefits of drinking wheatgrass juice, in spite of scientific evidence (and common sense) poking holes in the claims made by (of course) the people who profit by selling the juice. After all, scientists must have some motivation for wanting us to remain ill, and the people selling wheatgrass juice couldn't possibly have a motivation to get people to spend more money on it.
So the next time you are making a decision, ask yourself on what basis you are deciding: are you using reason, or basing your decision on fear?