Why is a dog on the cover of The Curious Incident of the Dog in the Night-Time?

Why don't you ask mark haddon on his website for a good answer?. the purpose was to introduce to people that people living with aspergers aren't so different from others and that the world needs to learn to accept that some people are different. haddon introduces the idea of the family life of someone with aspergers syndrome well and captivates the audience with a hard hitting tale of a search for acceptance.