Christopher is autistic; people with autism have specific likes and dislikes about many things. They need structure and continuity. There is no reason for this. I work with autistic children everyday. Some of them are particular about keeping the notebooks in a certain order, others won't hand in a paper in which the penmanship is not impeccable, others will only wear the color blue, and still others can tell you every detail about their favorite cartoon but notice nothing about the house they live in.
Christopher likes dogs because he knows what to expect from them; he dislikes certain colors just because, he enjoys math because it's rote learning and therefore there's nothing to be understood. He can't inference, but he can figure. Life is black and white, there' isn't an inbetween............ he likes schedules, breaking the norm is frustrating. I could fill this page over and over again with reasons, but no one can give you specifics.
I hope this helped you a bit, and I hope the novel gave you some insight.