it’s showing bias because the author uses the word “all,” as if all dogs that exist are actually mean. this is definitely not true, since a lot of dogs are really nice and friendly and will not harm anyone unless provoked. the author has probably encountered a few dogs before that have not been super friendly and has had a bad experience with them so now they believe that all dogs are bad. therefore, the author cannot for sure say all dogs are mean without sounding biased, as they have no evidence that shows all dogs are the same way.