Some good reasons to have your pet spayed or neutered are:
MALES: Male dogs who are not neutered will be more likely to roam and mark their territory with urine, mount toys and guests and to be aggressive. Dogs who are left un-neutered have a good chance of developing either prostate or testicular cancer later on in life.
FEMALES: Studies have shown that having your female dog spayed before she goes through a heat cycle greatly reduces the chance of developing mammary cancer later on in life. All un-spayed females run the risk of developing a serious infection called PYOMETRA each heat cycle they go through. Left untreated pyometra is fatal. The treatment for this disease is an emergency spay which can cost two to three times as much as a routine spay.
One obvious complication of not having your animal spayed is PREGNANCY. Although it may sound appealing to have puppies running around the house most people do not consider the expense and effort required to look after these little creatures. Some may refuse to nurse their babies, leaving the owner responsible for feeding and taking care of the babies every 2-4 hours around the clock.
Spay/Neuter is overall healthier for the animal when acquired as a pet.