Why do cats run away from dogs?

Not all dogs chase cats, but there are several reasons why a dog would chase a cat. Cats that are let outside usually roam, as they have quite large territories. Dogs, that perceive the house they live in as their territory, might chase off a cat if the dog feels like its territory is being threatened.

Some dogs have a higher "chase" drive than others, and will just chase animals smaller than themselves for no other reason than to chase it. These reasons do not mean that dogs hate cats. Dogs and cats living in the same household are often very friendly towards each other.