1. No risk of bringing unwanted puppies into this harsh world. Awhile ago, I found a controversial news regarding the PETA and ASPCA regarding the number of dogs and cats they euthanize yearly. The author made it a point that approximately 90% of the dogs rescued are put to sleep. I'm not sure if this is real, but hearing how many of these innocent doggies are being put to such an ordeal is heart breaking.
Be a responsible owner. If you don't plan on taking care of puppies, neuter or spay your pet to prevent overpopulation.
2. Lessens aggression. According to some books, altered dogs are calmer and less agitated. This also means that the males are less likely to pick a fight with other dogs.
3. Surprisingly, you'll no longer need to worry about the mess your female dog's monthly period will cause.
4. Lessens the risk of dog cancers. This includes female breast, ovarian, uterine and cervical cancer. For the male dogs, prostate infection and testicular cancer.
5. Female mood swings. Don't forget that dogs have hormones, too. And they can act the same way as your daughter or wife, with mood swings. Spaying your dog will lessen the tendencies of mood swings.
There are some speculations saying that spaying an aggressive female can even worsen her aggression, I suggest to consult a dog trainer or a canine behavior consultant to see if her aggression should be worked on before spaying.
Altering your dog doesn't change his character. Spaying and neutering only takes away the innate desperate instinct of your dog's breed.