Danger of Not Spaying or Neutering Your Pet

Every year, millions of unwanted dogs and cats, including puppies and kittens, are euthanized. Spaying and neutering prevent unwanted litters, help protect against some serious health problems, and may reduce many of the behavioral problems associated with the mating instinct.

Early spaying of female dogs and cats can help protect them from some serious health problems later in life such as uterine infections and breast cancer. 
​Neutering your male pet can also lessen its risk of developing benign prostatic hyperplasia (enlarged prostate gland) and testicular cancer.

This information was summarized from AVMA's article here.