Castrating or neutering a male dog removes the dog’s source of testosterone, which has been associated with numerous unwanted behaviors. It can take much longer for the effects of these hormones to decrease.
Changes After Neutering
For most dogs, testosterone has been linked to problems ranging from roaming to urine marking. Testosterone has been linked to greater reactivity in dogs, meaning that intact males may be more aggressive. Neutering will not immediately change any of those negative habits. Most neutered dogs will show improvement, especially if they were not already sexually active before they were neutered. Some dogs will still retain traits like urine marking, although most dogs, once castrated, will show behavioral improvements. Traits not associated with neutering won't change in your dog.
Benefits and Risks
Population control is a major benefit of castrating dogs. As mentioned, neutering dogs can help significantly with behavioral problems, although it is not a cure-all and training is still needed. Health problems, including testicular cancer and prostate enlargement, can be eliminated by neutering your dog. Anesthesia has risks, although careful monitoring and evaluation prior to the procedure minimize these risks. Neutering young dogs also appears to be safe, with limited side effects, although some correlation has been presented to neutering at a young age and development of certain cancers -- including hemangiosarcoma -- and cruciate ligament tears.