Decrease or Remove the Chance of Cancer in Pets
Spaying your pet before going into its first heat cycle will remove all chance of ovarian cancer and greatly decrease (greater than 99%) the chance of breast cancer. Every time a cat or dog goes through a heat cycle they increase their chances of breast cancer later in life because of the increased estrogen surges during the heat cycle.
Neutering your pet at any time will remove the chance of getting testicular cancer.
Decrease Chance of Secondary Hormonal Induced Diseases/Problems
Spaying stops your pet from having heat cycles. A heat cycle is the time the ovaries prepare eggs to become fertilized. In dogs, heat cycles occur generally every 6 months. The vulva becomes engorged and can bleed for up to 14 days during this time. In cats, heat cycles are induced by intact males in the area and can go in and out of heat on a weekly basis. This can become very annoying for most owners.
Pyometra (an infection inside the uterus) is greatly reduced with spaying. With each heat cycle, the cervix opens to allow sperm into the uterus which can also allow bacteria into the uterus. When your pet is spayed, the ovaries and the uterus are removed preventing your pet from having a pyometra.
Intact dogs can have a chance of developing testosterone induced peri-anal adenomas (growths around the rectum). If left untreated they can become more aggressive and metastasize into the bone. Testosterone will also induce prostate problems as well causing constriction around the urethra and difficulties urinating later in life. Neutering will not only prevent these problems, but will help decrease symptoms once they arise.
Decrease Marking/Urination in the House
Urine marking is your pet's form of communication between animals. "This is my property so keep your paws off it." However, what they may consider their property may be what we consider as ours: the couch, the speaker, or a dresser. In-tact males tend to be more possessive than neutered.
Urine marking is also used to advertise that your pet is available to be a mate. Neutering will greatly reduce your pet's testosterone and greatly reduce the urge to mate.
One additional observation about male cat urine is that intact males have a much more pungent urine smell.
Decrease Aggression to Other Animals
While neutering will not completely remove all aggressive behaviors, it should help. By reducing the testosterone level in males, pets are less possessive of their territory and tend to be less threatened by each other. If your pet is having aggression toward animals or people, you should consider neutering your pet as well as training.
Females are not as territorial as males, but can become very aggressive while in heat, especially cats. Spaying will stop heat cycles and keep hormone levels down decreasing their aggression.
Decrease Desire to Run Away
Intact males can smell a female in heat from several miles away. Dogs have a special feature in the bottom of their nose to detect pheromones called the Jacobson organ. These pheromones are released by females in heat to let males know they are ready to breed. The desire to breed can be overwhelming and cause even a very obedient dog to run away and find a mate.
Neutering will not remove the Jacobson organ, but the decrease in testosterone will decrease their desire to breed. So they may know a female is in heat, but there will be no desire to run away to her.
No Unwanted Litters
Even if you only have female dogs, there is a chance they can become pregnant by dogs in the area that can smell when they are in heat. If you are not prepared to care for puppies, then you should get your pet spayed. Puppies take space, time and a lot of care and should not be seen as something that would be a lot of fun without thinking about the care involved in raising them.