Rabbi Eliezer Hagadol stated that one who raises dogs is equivalent to raising pigs, and he therefore is included in the curse of the Sages. The Shitah quotes Rav Yehonasan who says that the opinion in the first braisa agrees that a dangerous dog may not be raised, but only due to the verse of ma’akeh (a fence), which states lo tasim damim – you shall not introduce blood in your house.,which was quoted on BK 15.
The Maharshal (BK 7:45) discusses why the prevalent custom in his time was for Jews to have dogs in their property. He first considers the possibility that since we live amongst non Jews, some of whom are hostile to us, we may raise the dogs for protection, just as the Gemora allows this for border towns, including Nehardea. He rejects this possibility, since even when kept for protection, the dog must be chained down during the day (when people walk around and may get hurt), and only let loose at night (when people are not walking around). The prevalent custom is to keep the dogs unchained even during the day. He therefore states that the Gemora’s statements on daf 15 and 83 are referring solely to a kelev ra – a bad dog, which can harm and scare people, by harming or barking. The Mishna therefore referred to one who raises Hakelev – the dog, i.e., the prohibited kelev ra – and not just kelev – a dog. However, our dogs, which are docile and do not scare or hurt people, are not included. Instead, they are included in the category of kelev kufri (80a), which Rashi explains as either small or docile dogs. People are used to these dogs, and are not even scared of them. Any dog that scares people – even if it cannot harm them – is forbidden, as indicated in the story of the pregnant woman.