Dogs come in all shapes, sizes, coat lengths, colors and dispositions. One area that tends to be overlooked is characteristics. Although many shelter dogs are a mix of breeds, the breed that can be identified from physical features often exhibits some of that breed's characteristics. For example, a mixed breed dog that has obvious features of a Border Collie may exhibit behavioral tendencies shared among its breed group: the Herding Group. And, more specifically, may reveal characteristics that are identified by that specific breed.
While you consider the characteristics of a dog that you will adopt, examine your lifestyle, living space, and time. If you work 10 hours a day, live in an apartment, and like to read or watch TV in your off time, a Labrador Retriever is probably not the best choice. However, a dog that prefers a more sedate lifestyle, like a Shih Tzu, might be the better option.
On the other hand, if you are an active family with children in the home a Lab might be more appropriate than having a sound or noise sensitive dog.
A toy breed as the Maltese might not be appropriate either because they are small and can easily be stepped or fallen on. Getting a dog on impulse, primarily based on appearance is not the best approach. Shelter staff can be very helpful in guiding you toward the right dog. They may discourage you from certain dogs but encourage you toward others.