Your understanding is very poor then, because I, like 99.9% of the other people in Tokyo have already made a "decisive decision" to stay as there is no reason to leave!
In denial about what exactly? Wouldn't I have to be in some sort of danger to be in denial? Your attempts at psychology are laughable. If you laugh at someone who runs away from a child with a water pistol then that is only because you yourself are in denial about your own fear of children with water pistols! The only tiny flaw in this theory is that, well, water pistols are not dangerous so there is no reason to be afraid of them. There has been no reason to be afraid of radiation in Tokyo and well, the vast majority of people, unlike you, aren't afraid of it. With your Junior High School cod psycho-analysis skills you should be able to get your head around that!
Hey, why aren't you taking measures against being abducted by vampire aliens? Uh-oh, looks like you are in denial!
I'll give you another motivation: because it is quite amusing. Seeing people panic in itself is not so amusing but seeing the subsequent tying themselves in knots of self-justification and trying to boast about their martial arts skills and the internet "beatings" they have been dishing out has been hilarious. A much needed tonic in the awful days following the devastating earthquake and tsunami.
I am certainly glad that you are going to give up reasoning. I think it is often a good thing when people decide to give up things they are obviously just not very good at.