Why do people ignore the obvious?

In our book The Plague rats keep appearing and dying in front of people yet no one tries to figure out why or connect it to the fact that people are starting to get sick.
So that brings this post about why people ignore the obvious?
I think everyone is guilty of telling themselves a white lie and believing it. I think we do it for our own good because we want to be happy. We tell ourselves that everything is ok because we want it to be that way and we think that of we continually say it then it will become true.
Maybe that could just be another level of fear. Just like the wives that tell themselves that they are happy in a marriage when they know they aren't.
I think that in the novel the people of Oran are afraid of admitting that the plague is happening because they know they are in a losing battle. From the deaths that have already happened suddenly the people know that death is approaching but refuse to believe it and instead trays to find different excuses for why certain people have died.