The Curious Incident of the Dog in the Night-time 2

I don't understand why christopher thinks that a certain amount of cars makes a day good or bad.  I also wonder what he does if there is yellow cars and red cars together. This reminds me of when sometimes if you dread something or  if you dont feel happy one day you get mean and your day gets worse because of a negative attitude. I can also agree with the psychologist about Christopher having a unlogical way of thinking for being a very smart person.