why did cat and dog always fight??

Cat and dog are very close to humans, but if they met, the situation became tense. The main cause is due to differences in the way they communicate and reason for the different body movements. Example : Dog  move his tail to show friendship, but cat move his tail to scare the enemy. And then the cat moaning noises (eeerrhh) to indicate a good impression, but dog considers it sound as an invitation to fight. If cats and dogs living together since childhood, understand how to communicate with each other, there would be no hostility :)