Here's my take on why vets push neutering:
Most neighborhood veterinary practices operate on very small margins. Vets take continuing education courses, and some of those courses are in veterinary practice management. Those practice management courses teach them to push two things really hard: vaccines and spay/neuter. Vaccines and spay/neuter are overhead procedures. If you get a steady stream of those, you can keep your doors open. These continuing education courses arm vets with all sorts of arguments they can use against (yes, against) reluctant clients to get them to commit to vaccinations and spay/neuter whenever they bring in a new puppy.
These courses also provide vets with data that demonstrates that they have to push vaccinations and spay/neuter hard beginning with the very first visit from a puppy. Statistics show that the longer a client waits, the less likely it is that the vet will get the revenue from these procedures. Either the clients won't have them done, or they risk having the client go somewhere else.
Neighborhood veterinary practices need to sell vaccines and spay/neuters to stay in business. It is often done much more for the sake of the veterinarian than for the dog.
Ever notice that some vets will push annual vaccine boosters really hard, but will balk when a client says they'd rather titer rather than vaccinate? That's because the vet gets the fee for the vaccination, but often has to send out the sample to be titered, and the lab gets the fee.
There is no medical reason to neuter a dog before two years old (or ever, really, for males). But vets push them as early as six months, when they know that the dog is immature and that a spay/neuter can be detrimental to the dog. The reason for that is money, pure and simple. The earlier a vet can convince a client to do a spay/neuter, the more likely they are to get that fee. Vets do not profit from waiting until two years old. Much of the time, a client that waits until their dog reaches sexual maturity will wind of never getting the dog spayed/neutered. So they push them early. For their own benefit, not the dog's.
Look around at the medical reasons why you should NOT spay or neuter a Golden retriever. The research is dramatic and compelling. But there is NO research that says it's better for the dog to be neutered early. Vets know this. So you have to give some thought as to what the motivation might be.