Because we have to.
No, not that we are forced to believe in a certain way, but that we must believe in a certain way in order for our society to function.
A doctor must fully believe in our medical system, or it's long term goals, in order to administer questionable treatments.
The system becomes more important than a doctor's own opinion and ensures the survival of that system.
Side effects to certain treatments?
"No problem, the long term goals of such a protocol will outweigh any negative effects on a relative few individuals", a doctor might say.
Without this thinking, a doctor would be more apt to become stressed.
A doctor would actually have to think before administering any treatment. (But this thinking would limit the "efficiency" of the system and retard it's growth.)
A doctor needs to believe that he is right.
Religious beliefs (as they are applied today) are required to ensure that large groups of people approach problems from the same perspective and seek the same outcomes.
Having the ability to blame a higher power for the negative outcome of one's choices also frees conscious individuals to get on with their day-to-day lives.
(Too much reflection on one's sins would lead to guilt, depression and then surrender.)
Religion is the ultimate tool to inspire or oppress people because it's dictates are not to be questioned.
It's rules are final.
Government is another belief system that requires the participation of it's subjects in order to function.
The governed must believe that theirs is the only good, righteous and valid system.
This belief demonizes any enemy and and validates itself in order to justify any atrocities.
This system's strength is in it's ability to bring more believers to it's cause and to destroy anyone who seeks to reign in it's growth.
Our monetary system is another construct that is reliant upon men to feed it's hunger.
Back in the day, people would barter.
I'd kill a cow for you to eat if you built my house.
But this system fell out of favor because there was no real way in which to quantify the value of one's labor.
A cow for a house may have been a good bargain for either man - but not so for a fish.
How many fish is a cow worth?
Money became a tool for measuring the value of one's work - that's all.
Since gold was somewhat rare (You couldn't just dig up money to pay your bills), didn't rust and was easily shaped to reflect a measurable amount - it became the favored metal in striking coins.
After we left the gold standard, faith became the standard.
It requires faith to believe that a piece of paper accurately measures the worth of your work.
It requires even more faith to believe that a few 1's and 0's can do the same.
The market (and our economy) didn't crash because people lost any real wealth - they crashed because people lost faith in the way we measure wealth.
But what happens now?
Do we believe that the system that has brought us this far is flawed?
Or do we blame Osama Bin Laden, illegal immigrants, BET, or Barrack Obama for our country's problems?
People who were taught that their system was right and just are the one's who seem the most stressed.
Those on the outside never relied on the system to justify their well being.
The whole Tea Party reaction is just the result of a few people who imagined that the Constitution only applied to their group.
Their reaction is just a disbelief in the fact that much of what they were taught was (is) wrong.
That they are not special or chosen by God to rule the world.
That other people are just as good as themselves.
They are just in shock that "Niggers", "Wetbacks" and "Faggots" are getting what they thought belonged solely to them.
They are just reacting as would a person suddenly waking from a dream.
But these beliefs are required for any capitalist society to function as a whole.
When these systems function properly we have a very small group of the "haves" and a very large group of the "have nots".
These systems keep people driving in their lane at the right speed and in the right direction.
These systems help us to engage other people and accept our fate.
These systems enable us to feel connected in our common dream.
Without these systems - we would see our nakedness.
We would become jaded and un-trusting.
Our society would crumble and everyman would be responsible for his own well being.