Some studies indicate that neutering a male dog will decrease their aggression towards other dogs. The reason male dogs become less aggressive is believed to be because it removes the dog's ability to secrete testosterone. Another reason why dogs may become less aggressive is because they no longer have to compete with other male dogs for mates.
There are many reasons why neutering a dog is beneficial to their health. It can reduce their chances of developing testicular cancer and it also prevents prostate problems. Neutering is also beneficial because it reduces the dog's desire to mark their territory with urination, prevents their need to roam in search of a mate and it reduces the dog's urge to inappropriately mount on other dogs, objects or humans.
The best time to neuter a dog is before they reach sexual maturity, or around 6 to 9 months. Waiting too long to neuter a dog can cause them to develop unwelcome habits, and it can be hard to break them from these habits.
Neutering a dog is no guarantee they will be less aggressive towards other dogs or that it will completely change their bad behavior. Dogs that still have aggression or unwelcome habits may need the expertise of a qualified veterinarian or certified dog trainer. Neutering does not remove all of the testosterone in the dog's body, which can cause some amounts of aggression to linger.