Just like it is more difficult to stand by your apples when you have no apples, rather than when you have many? For instance, we get attached to our material possessions and do not want to give them away, same goes for our preconceived notions. If we did not value our material possessions we would not mind giving them away. Obviously when we have no material possessions, we have nothing to value, thus there is simply nothing for us to cling to.
For this reason a most open-minded examination is most easily conducted when we have as few ideas as possible that we would be reluctant to renounce.
For example, if I were to make an argument as follows. If A then B. A. Therefore B.
If the premises are true, the conclusion must also be. This is a requirement of any valid argument, I am certainly curious about how it is possible to analyze something without being led to believe that this or that thing is true in the end. Reasonableness requires you to believe an argument which you cannot declare invalid or in which you could point out the falsity of the premises. Of course though, you can declare yourself unreasonable and believe in anything you want! Your remark regarding that postmodernists do so is quite pertinent on that note!