Humans bred breeds to use them in sports for centuries. Many instincts that were at first used for work, guarding the herd... were also abused. Dogs and other animals were becoming victims of human entertainment.
Greyhounds are bred for races because of their speed. Besides them, other go to breed is Whippet, as well as other sighthounds that can be seen on the racetracks. before chasing after a mechanical rabbit, greyhound was used – as well as other sighthounds – in hunting wildlife, which is probably one of the oldest sports in which dogs are being used.
Cruel human wit is advanced, so a type of "entertainment" was invented where shepherd dogs chase after sheep, and in Roman time, battues for bears and bulls became very popular and stayed that way until late 19th century. Battues for badgers that will end up torn by dogs entertained the Brits.
Humans put animals in cruelest sports that they invented. Participants in bloodsports, throughout history, were roosters, monkeys, bears, bulls, dogs... Bear-baiting and bull-baiting were popular until 1835 when they were forbidden. Unfortunately, as one misdoing was ceasing, other was progressing – dog fights were becoming popular, which encouraged breeders to create breeds fit for fighting and many other breeds were modified and used for that part, even breeds whose primary purpose was something else, due to suitable features, were very often used in fights. Dog fights were not forbidden until 1911, but we are witnesses that this law, as well as the one from 1835, does not stop humans, even today, to entertain themselves in such "wonderful" way in USA, European countries, Asian countries and many others around the world.
Some support theory that fight dogs like to fight and that it is in their nature, that it should not be denied. But that would mean approving of deviant behaviour of such dogs. It is not natural dog behaviour. It generated thanks to human doing – abuse of animals. By interbreeding they created ones that will entertain them and make them money while they slaughter each other.