
I would say the factors fall into two categories. There are the personal ones, and those related to the breed.
Under personal I would put, cna you cope with the changes in your dogs attitude to the world and other dogs (he may bec0me more inclined to fight, wander, and pee in the house) after he has been used? Can you accomodate visiting bitches? Can you take in your dogs offspring if they can no longer be kept by their owners and the breeder is unable to do so? Will you know how to manage a mating? Will you be available any timne that the bitch is ready?
The Breed ones are I would say: Is your dog an outstandiong example of the breed? Has he proven himself in the showring/working arena as being superior to his peers? Does he posess bloodlines that are rare or would be lost to the breed if he were not used? Does he have brothers or other male relatives that have better qualities than he does. Do you know enough about the finer points of the breed and breed standard to answer the above?
I am sure others will add to the items I have listed, as these are just those off the top of my head.