It's sometimes difficult for new dog owners to decide whether they want to spay or neuter their dogs. Although I am a firm believer in spaying and neutering, I think it's important for dog owners to know both sides of the argument, and to weigh the pros and cons of the procedures.
Pros to spaying or neutering your dog:
It eliminates the risk of pregnancy
Dogs who have undergone spaying or neutering are unable to get pregnant, or get other dogs pregnant. No matter how well many dog owners think they are looking after their dogs, there is still a possibility that they may get away and either become impregnated, or get other dogs pregnant. With dog overpopulation already such a problem, it would be terrible to unexpectedly bring more unwanted puppies into the world who may or may not find good homes, increasing the number of dogs that may end up neglected, abused or abandoned. Not to mention the fact that an unplanned and unwanted pregnancy would put either your dog or another through the ordeal of giving birth when it wasn't necessary and could have easily been avoided.
It reduces aggression and unwanted behavior
This point should not be misunderstood as meaning spaying or neutering would negatively affect your dog's playful or hyper personality—your dog will remain as active as it was born to be. However, spaying and neutering can help eliminate particularly aggressive or unwanted behavior that dogs tend to exhibit when they're particularly hormonal.
For example, un-neutered dams or bitches (female dogs) will have a bloody discharge when they go into heat, which can become messy if not cleaned up properly by the dog herself. The way dog will to clean herself will be by licking constantly and for long periods of time, behavior that can become irritating to a dog's owner. Dams in heat will also be interested in attracting dogs of the opposite sex by urinating a lot, which I think most dog owners can agree is an undesirable behavioral trait. Having sires (male dogs) constantly approaching a dam can also be frustrating if there is no intention to get the her pregnant. It means you would not be able to bring her to dog parks or areas where other canines are roaming about potentially running into a situation where you need to break up a mating session.
For dams and sires (male dogs) alike, when they're in the mood to mate, they can become extremely aggressive, to the point where you could be bitten if you try to interfere. A surge in hormones and the desire to mate can also result in your dogs taking it out on other dogs, your furniture, and anything else in order to expel some pent up energy. By spaying and neutering your pets, you eliminate or at least reduce this kind of aggression.
However, don't worry, if your dog is naturally playful and hyper, they will still exhibit those characteristics after the procedure.
It eliminates the risk of certain diseases
Dogs, like people, are susceptible to various diseases, including cancer. By spaying or neutering your dog, your dog will not be able to contract viruses, bacteria and cancers specific to the reproductive organs. For dams, neutering not only ensures she will never mammary cancer or tumors, the inability to get pregnant will also eliminate their potential to become injured from undergoing an unwanted or dangerous birth. For male dogs, spaying makes sure he will never experience cancer or tumors and growths on his testicles.
Cons to spaying your neutering your dog:
There may be surgical complications during the procedure
This is really the only real "con" I think can of regarding spaying or neutering your dog. Depending on the breed, dogs can be vulnerable during surgery just like people can be. Specifically, some dogs do not react to anesthesia as well as others. For example, surgeries performed on short-nosed dogs like Pugs and French Bulldogs can be particularly dangerous because of the nature of their respiratory systems. This is why it is important that you have a thorough discussion with your vet, and make sure the doctor knows the complications surrounding performing surgery on certain types of dogs. Certain dogs may also have allergies to certain medications and anesthesia, which is why you should also do your own research to decide whether you are comfortable spaying or neutering your dog.
Dogs will experience discomfort and pain afterwards
It is awful to see your dog feeling any kind of discomfort or pain, but unfortunately this is inevitable following a surgical procedure such as spaying or neutering. Most veterinarians are able to prescribe pain killers to treat the pain until a dog is healed enough that she or he no longer needs them, and most dogs usually recover quite quickly and are back to their old selves within a few days. However, if you're the kind of person who absolutely can't stomach this idea, then perhaps spaying and neutering is not for you.
There are also many myths surrounding the spaying and neutering of dogs, such as the idea that dogs will gain weight after the procedures or that the surgery is too costly. These ideas are frankly not true, and dog behavior expert Cesar Milan has a great web page specifically addressing these myths.