Why do we believe in the necessity of belief?

In conversations, I often hear the emphatic phrase “I believe in …” referring to any controversial or improbable subject. For example, I hear “I believe in life after death." This is usually someone who has not experienced near death, but has decided based on second hand information that the case is plausible enough to commit to a belief in it. In the absence of knowledge, belief is declared. Those who have actually had a near death experience tend to say, “I know what I saw.” In other words, they are beyond the need for a declaration of belief.
It seems that even among people who are not religious, there is the belief in the necessity of belief. There is even: “I believe God doesn’t exist." Why are we so programmed against a position of ongoing uncertainty, of a confident “maybe?” The mind has plenty of shelf space for various contradicting arguments. Why can’t we hold multiple positions and accrue evidence for and against all of them? Yes, each position reflects the ideas of its opposite, but too firm positions close the mind.
I recognize that in many cases a strong belief, regardless of evidence, is highly motivational and therapeutic. Great mosques and cathedrals are evidence that belief can propel ordinary people to create the almost impossible. And there is healing power in belief as well. But this is only an argument for the function, the power of belief, its usefulness — not its necessity.
When it comes to the ultimate nature of existence, I know that I don’t know and I assume, but don’t believe, everyone else also doesn’t know. I am perfectly at peace with that.

I love this interview with Bertrand Russell where he says: "... and if you can't find out if it's true or not you should suspend judgment". Why is this "suspension" such a taboo?
http://www.youtube.com/watch?v=XrxSkc68Ci0

One more link: Larry David:

"If I really believed that stuff, I'd keep it to myself."

http://www.youtube.com/watch?v=z1yOckwMm_k&feature=related