It's mean to hate someone's dog.
Bob's neighbor has a dog and it has been using Bob's yard to as a bathroom.
Bob wants the neighbor's dog to stop going to the bathroom in his yard.
Bob lets his neighbor know that he has been cleaning up after his dog and would appreciate if he would not have to do that.
Bob tells his neighbor that he can't stand the dog and if he sees it around his yard again, he will call animal control to take it away.
Bob's neighbor responds with some embarrassment and promises to take care of the problem.
His neighbor responds to his words with more anger and never talks to Bob again.