All humans are mentaly ill.Well sort of.Since there is no "correct" standard of mental fitness to which humans can measure themselves its dificult to know what is sane.
Most people do or think things other find strange, in fact I have yet to meet anyone who does not do at least one thing I find strange, and I expect people would think the same about me.
But the real problem is that humans are delusional.We believe or think things that arn't true.
You have crinimals who justify what they do to others in their minds and won't admit they are wrong.Greedy Business men who reason that they are not harming anyone by their greedy ways.People going to auditions to be a pop star who think they can sing when they cant.People who follow the Jehovahs witness organisation no matter what.In fact how many of us can really admit to our flaws and weakness when we think we are doing a good job?That is the important part , when we think we are doing right or want something we shouldn't have and justify it , we are deluding ourselves.We are showing a sign of irrantionality in our judgment of our actions.
Its hard to go through life without justifying what we do.Those who always think they are wrong tend to end up going mad anyway.
But it seams to be a function of the brain to delude ourselves when we need too.The brain lies to us.This makes us act irrationaly and prevents us from considering the real facts.