Dog’s neutering is important not only for the population control but also for some other good reasons, like genetics improvement (preventing dogs carrying harmful genetic traits, such as the common  hip dysplasia or epilepsy, from spreading them) and  the positive effects that castration has on the animal’s health.
One of most important positive medical effects occurs in the prostate which, because of the presence of testosterone, will enlarge gradually during the course of the dog’s life. With aging, it is probable that the prostate enlargement (though benign) will bother the animal, even affecting the defecation. The presence of testosterone also increases the possibility of development of other diseases, such as cysts and infections, involving the prostate. Neutering results in prostate shrinking thus preventing the benign hyperplasia and other diseases, like certain hernias and testicular and perianal tumours, that frequently affect this gland.

However, and contrary to what is generally believed, neutering does not reduce the danger of prostate cancer.

What behavioral changes may occur after neutering? What behavioral changes may occur after neutering? The only changes to be expected are those related to behaviors which are influenced by the presence of male hormones. Desirable behaviors towards humans like playing, docility and socialization remain unchanged. In fact, the behaviors that change are precisely those most unwanted. Roaming is reduced in 90%. Aggressiveness towards other dogs and also people is reduced in 60%. The urine-marking of territory is reduced in 50%. The total sum of unwanted behaviors is decreased by 70% in castrated males.

Early neutering
The surgery can be performed at a young age (8 weeks onwards). Early neutering does not cause any appreciable differences in physical, skeletal, or behavioral development when compared with animals neutered in the adulthood, although dogs castrated before the puberty (generally 6 months) tend to grow a little more. Animals neutered at a younger age often have faster recoveries than those neutered when they are older but old dogs should also be castrated in case of prostate enlargement or other prostate diseases, subject to careful complementary medical examination of the dog’s overall health before surgery.

Overweight?
The level of activity and the appetite do not change with castration. There is no reason for a neutered healthy dog to become obese unless he eats too much or is not daily exercised as he should be. If both, eating and exercising are duly controlled and the animal keeps gaining weight, he should be examined for heart, thyroid or other metabolic disorders.

Will the dog still be interested in females after neutering?
His interest will be reduced but it can be aroused by females in heat.