I didn't create the dogs together in cas im not sure if that matters... but every time my sim suggests they breed the one dog goes up to the other and they do a thing like growling at each other and...
You can use the "Be nice to" interaction under the "Friendly" menu to get the dogs to like each other more. You're also more likely to be successful if you are companions with the animals (I don't know if you need to be companions with both animals or just the one that you encourage to breed). To become companions you need to get your relationship with the animal up quite high (I'd say 85%+), then it will become an option under the "Friendly" menu.
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