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Service dogs can come from many different backgrounds. Some are intentionally bred for service work, some are career change dogs bred for other work, some are rescues, and yes, some started out as pets.
However, the odds of any given dog having all the "right stuff" and actually completing training to become a service dog are about 1 in 100. The vast majority of pets, even lovely, well-behaved pets, aren't going to be suited, though it is possible.