See, I disagree, I think that humans start out in a state of wonder about the universe, open to every new idea and opportunity. Then, they learn theories that explain the world, thus making them close out other theories that conflict too much with their own. I think that fanaticism is the result of believing in your theories so fiercely that you cannot consider others, that is, you will destroy other theories so that yours will not be conflicted. This seems more like a move of desperation to me, something one would do without thinking when they are overwhelmed. It is not in our base state that we are fanatics, but in our most distressed or most emotional state. This can easily explain how anger or desperation can cause fanaticism, but then, what about isolation? One possibility is that it is the jump from isolation into large-scale contact that overwhelms people, but there is a problem: that does nothing to explain the fanaticism beforehand. Thus discarding the jump theory, I think that it is the excess of thought due to isolation that causes fanaticism. As one spends a large amount of time thinking about only one theory, it solidifies in their mind. I argue that this solidification is what causes the fanaticism .
Now the fanaticism from isolation has very different properties that the emotional one. First, isolation causes the long-term solidification that is nearly impossible to undo, while emotional rushes can cause dynamic solidification, it is neither stable nor long lasting, thus it will usually revert. I say usually because perhaps in special cases, such as deep hatred or anger, the emotion can solidify into the person, along with the theories it supports, thus creating a fanatic much like the isolated one. The dynamic fanatics (emotional rushes) can be extremely dangerous in an instant, but the solidified fanatics have the capacity for much worse over a longer period of time.
So what forces in society today create fanatics, and specifically what types?
Dynamic:
Movies/music/mass media/popular culture
Pressure/stress
Intense situations
Solidified:
Single dominating emotions (Ex. greed, love, etc.)
Religion
Isolation
So now that fanaticism has been examined, it raises the question: how can we make sure to avoid it? First, I would say that the only way to truly avoid fanaticism is to not care or think about anything. Since that is both unrealistic and inhuman, I would suggest revising the question to deal with solidified fanaticism, as it is far more detrimental to a person's future. The first defense is to be intellectually stimulated. The introduction of new knowledge into a system will help to prevent solidification, especially when it conflicts with personal theories. Second, human interaction helps to prevent solidification in a similar manner, but only in an open interaction, that is, only if both parties have not yet solidified. If one party's mind has solidified, it will not be able to interact with the other, thus creating a dead exchange, which would end up solidifying the other mind. Lastly, be sure to accept the uncertainty of your personal theories, whatever they may be. Uncertainty does not make a theory false or incorrect; rather, it is a simple reminder that your knowledge is not absolute. Accepting this uncertainty will prevent solidifying of any theory, as they all have their element of uncertainty.