I think between the definite cases of conspiracy that get exposed in the media, and the many novels and movies that have conspiracies as their central theme, we have been conditioned to suspect the worst. It seems natural to look at a situation, see who benefits from it, and then assume that the folks reaping the benefits were involved in a conspiracy to create the situation. For instance, the 9/11 attacks led to the administration's ability to coerce cooperation from Congress on economic, political, and military options that would have otherwise been stalemated. Those same attacks also benefited the various companies supplying the war effort, so some folks point to that and hypothesize a conspiracy between the war mongers in government and those companies. As noted earlier, with the large number of people that would be involved in such a conspiracy it would be highly unlikely it could be kept quiet. But there is always that little bit of doubt.

One thing that I truly believe is that our system of government has become stagnant, with true change only possible as a reaction to crisis (for instance, without 9/11, do you really think Congress would have gone along with the administration's assault on privacy?). There are many in government who also believe this crisis theory, and it is not totally far fetched to believe that some group or groups would create economic or political havoc to advance their agenda. The world is no idyllic place where good automatically triumphs over evil and the majority always does the "right thing". In my opinion, just the opposite is true. The world has become a more dangerous and evil place, and healthy skepticism on the part of it's citizens is necessary.
Jerry