If you have no intention of breeding from your dog there are many good reasons to having them neutered
Neutering eliminates the occurrence of testicular cancer. Neutering markedly reduces the incidence of benign hyperplasia of the prostate gland, prostatitis, prostate cancer and perineal hernias in dogs. Male dogs display hormonally influenced aggression toward each other, Neutering eliminates much of this behavior without affecting a male dog’s protective instincts towards his house and family members. Neutering will often decrease or eliminate other objectionable male dog behaviours, such as mounting furniture and family members. Male dogs will cease roaming to find a mate because the hormonal urge to do so has been removed.
Spaying eliminates the bloody discharge of female dogs in heat and removes the risk of unwanted litters. Spaying eliminates false pregnancies and the development of pyometra (septic uterine infection) in dogs and cats.
Spaying when young also reduces the incidence of mammary cancers.