Cats and dogs fighting is not an uncommon sight. Because they come down from different species, a cat cannot tolerate a dog's goofy and playful attitude while a dog cannot stand a cat's royal behavior. It is a common belief that a cat and dog are not the best of friends. That is why you may often hear people say, "fighting like cats and dogs." Now the question is, 'why do cats and dogs fight?' Certain factors play a big part in the enmity between cats and dogs. Let us explore them below.