Actually, my dog gets along with our neighbor's cat very well. Every time the cat comes into our house my dog gets a cookie (and the cat some cat food) so my dog is quite happy to see the cat. They are both polite to each other, but I know this is not always the case.

Dogs were wild animals for a much longer period of time than they have been domestic animals (or tamed and kept as pets by people). They like to chase small creatures, that is part of their hunting instincts, so it is not that a dog is being bad, he is following his instincts and besides, he may just find it plain old fun. On the other site cats are also predators. Both are also territorial animals - in the wild, often seeking the same prey. This puts them in direct competition with each other in many cases. This could be part of it. Especially when they are similar in size. Dogs and cats also communicate very differently - this miscommunication can cause some difficultly with their relationships in the household. In the household, dogs are often larger, usually more energetic, and louder than cats vocally. These are all frightening to the feline -- it takes great patience for a very proud cat to warm up to those silly dogs.