Mammary cancer is very common in older intact females and is the most common cancer to spread to the lungs. Early neuter reduces the incidence of diseases of the prostrate, testicles, and other tissues influenced by male hormones. Testicular and perianal gland cancers are the second and third most frequently diagnosed tumors in older intact male dogs. Behavior is adversely affected by surgery. The only change in behavior is positive.
Male cats tend to reduce territorial spraying, depending on their age at neutering.
Neutered dogs and cats fight less, resulting in fewer battle scars, contagious diseases and abscesses. They also wander less because they aren't as interested in pursuing females in heat. Animals become fat and lazy after spaying or neutering. In fact, one unaltered male can be responsible for impregnating dozens of females. I don't need to spay my dog because she is always kept in the backyard.