As a US veterinarian, I hold a nuanced view of spay/neuter. Nevertheless, I generally recommend it as default for most pet owners, as it goes along with their expectations of what their pet's life will be like. People who elected to leave their dogs intact are often shocked by something down the line.
1. Pyometra. Dogs do not have menopause, but they may stop cycling. When they do they can develop a uterine infection that can become life threatening in hours.
2. Perineal hernias. Unneutered, aging dogs are at risk for pelvic floor weakness that results in specialty surgery (and neutering), costing thousands of dollars.
3. Hit by car. Unneutered pets are much more likely to escape randomly and get hit by cars. Even pets that normally do not run away from their owners.
4. Mammary tumor. Unspayed or late-spayed females are at significantly more risk than those spayed at 6 months.
5. Pregnancy. Shocked pikachu face when the unspayed chihuahua actually did get knocked up by their friend's unneutered chow.
6. Bleeding. All bitches bleed during proestrus.
7. Behavior. The dog-park behavior people like in their pets is the behavior of neutered dogs. Unneutered dogs are much more likely to attempt to dominate the other dogs and get into fights.
That said, it's a trade off. Unneutered males definitely have a different growth pattern and develop a stockier build. They also seem to have significantly fewer issues with the cranial cruciate ligament in the stifle (knee). There are some other, more hand-wavey associations of cancer in golden retrievers related to neutering, but it's hard to tell if that's just an artifact of the data as it's more likely unneutered dogs appear at teaching hospitals for injuries (hit by car, fights) rather than for cancer.
I would love to just recommend people be responsible adults. I wish that the US could be like Norway, where people don't seem to abandon their unneutered animals, and they don't seem to have problems with their bad behavior.