按照给定的主题，写一篇文章。
主题: 犬类在人类家庭中扮演的角色。
Dogs have been living with humans for over 10,000 years, and they have become one of the most loyal and beloved companions to humans. In modern times, dogs play a crucial role in various aspects of human lives, both personally and professionally. 

As pets, dogs have become part of the family. They offer unconditional love, support, and friendship to their owners, and they bring joy and happiness into people's lives. They are also a great source of entertainment, as they love to play with their owners and make them laugh. Moreover, studies have shown that dogs can provide therapeutic benefits and help reduce stress, anxiety, and depression in humans. 

In addition to being companions, dogs also have professional roles. They are used for search and rescue missions, as they have a superior sense of smell and can locate lost individuals. They are also employed in law enforcement agencies to detect drugs and explosives, and in the military to detect and attack enemies. Dogs can also be trained as therapy dogs and visit hospitals, nursing homes, and schools, where they help improve the emotional well-being of the people they visit. 

Furthermore, dogs are used in various sports and competitions, such as dog shows, agility, obedience trials, and flyball. These activities not only showcase the breed's abilities and talents but also provide fun and relaxation for both dogs and their owners. 

In conclusion, dogs are an integral part of human lives, and they play many roles in society. They offer companionship, support, and joy to their owners, and they also have a professional role in search and rescue, law enforcement, therapy and sports. With all the love and benefits that dogs bring to humans, it's no wonder that they are called "man's best friend."