Why do people use selective breeding to change characteristics of plants?

Selective breeding is done to increase the likely hood of certain traits desirable by the breeder. If a dog breeder liked the shape of a dog's ears, she would mate that dog with another dog with similar shaped ears. Incestuous mating occurs quite frequently in animal breeding due to the increased genetic similarity. All dogs belong to the same species, and therefor can produce viable offspring. The reason a Chihuahua and a German Shepard are so different is because of the ancient practice of selective animal breeding.