A simple and direct reason is: Beliefs, no matter they are with right or wrong result, usually hinder constructive changes that are based on facts.
Beliefs lead to ideology without factual support will bring unnecessary suffering and disaster to other living organism in the planet. In past history of mankind, beliefs were mostly religions as Catholics, Christians, Muslims, Buddhist. Beliefs in modern term also should include all kind of ideology not based on facts. One example is communism, they believed human nature would be contented to contribute their best and only take back what they need. So a communist society neglects the brutal, selfish and animal aspect of human beings. The tragedies of communism execution in Russia, China, North Korea, etc prove this ideology is very much deviated from facts beyond any doubt.
The other example is Environmentalists. They conceive that climate becomes warmer (still a controversy in science) only because of human activities. And they are always promoting the idea that Polar bears go to human community to find food is because global warming and the bears do not have enough snow and ice to live on. They choose to ignore the fact that population of Polar bears increase 2 to 3 times after the 50s because illegal hunting was banned since then. They see more Polar Bears wondering around is not the result that the bears is starving owing to not able to find food on land without ice covering (this thinking is already quite strange, without ice on the ground, should be much easier to find food for all kind of animals even the Polar Bears). This is the result of over populated Polar Bears, so many of them need to find food away from their original habitation. Again, Environmentalists perform what they do best, confusing correlation to causation.
For certain, those who embracing beliefs tightly are Creationists. Creationists are always asking this question, you have your belief and they have their belief, why some Scientists are so against religion. The danger of beliefs is that people with beliefs ignore facts and rational thinking. This attitude perhaps is good for human in ancient time. It helps to make decision fast no matter it is right or not. We know that in some life or death situation, fast decision (maybe it is proven wrong later) still get a better chance of survival (when not counting about the quality of survival). Our genes would not care if we lost both of our legs and arms, as long as we are still able to reproduce and can have descendants, then the survival mechanism (even it causes great pain and suffering) is still a better strategy for survival neglecting the quality of well-being.
The experience of inhabitants in Easter Island could show how the beliefs of dead ancestors lead to cannibalism:
In daily life, we may still utilize belief to facilitate our decision process and to make lives easier. But we need to be very cautious not to mix beliefs with facts. Take for an example, we can always choose not to bring one of our friends to picnic because we "believe" every time we bring him along, it rains. However, we have to distinguish that the phenomenon is just co-incident and is nothing to do with causation. With this kind of understanding, we can still live happily although we know in our heart that this belief is just a "joke" to make us feel comfortable. But if this belief is not controlled well, it may lead to sufferings too. Say, when one day a drought in our area happens, this belief may encourage believers to pull this friend out to attract rain no matter he is willing to go out or not. If by then, he is not able to bring along any rain, we may even angry with him and think he is not co-operating and terrible things on him may follow ......
After all, there needs not an explanation to everything we don't understand yet. Could we, human just humbly say that "We don't know the answer yet"? rather than making up a supernatural creature who has, and is all the answers. Admitting we don't know should be much better than giving an irresponsible wrong answer from beliefs that are rooted from a primitive evolution misfire in our human mind.
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