Why a dog is our best friend? When mentioning about human's best friend,you would say that it's a dog.Nowadays more and more people keep dogs as their best friends.The reasons are as follows: Fristly,dogs are very loyal to our human.And that's the reason why we keep dogs to watch the door in our home in old times.When a dog sees a stranger breaking into our house,he will bark or even bite.With a dog,there would be little chance losing our possessions.Secondly,a dog is a good companion for many housewives and old people.Since many people,like unemployed women and retired people,are very lonely at home,a good companion plays an important role in their life.When they feel lonely,they can talk and play with a dog.It will bring much pleasure to their boring life.Moreover,a dog can be easily trained.Then a trained dog can help people do what they want.For example,in Wenchuan earthquake,people used the smell of a dog to help find people who were buried in a pile of rubble. As stated previously,a dog is a loyal, helpful animal and he is a good companion for human.So that's why we treat a dog as our best friend. (This is my first composition.If there is any error,please correct me.)