While I agree that labs and goldens are generally very friendly towards other dogs, I want to point out that other dogs often do not like them as well. Because many of the retrievers (And other breeds, I'm sure; these are clearly just the most common.) tend to remain rather "juvenile" throughout their lives, other dogs can be very reactive to them. I know a lot of dogs, mine included, who hate when an adult retriever comes bounding up to them. It is behavior that she will happily tolerate from a young puppy, but not an adult. These happy, friendly dogs are not really following doggie social rules. I think that's what sets off a lot of fights at dog parks. Friendly retriever bounds up to another dog, entering into their space inappropirately, that dog tells them, "Back off and grow up", and things escalate. I'm very careful about taking Meg into parks because of it.
It may never be an issue if you have a bunch of retrievers, but if you want to add other types of dogs to your pack, you may want to keep it in mind.