Neutering is particularly effective as a preventive measure against a number of common behavioral problems.

Hostility toward Other Dogs
One aspect of male canine behavior is aggression towards other males. As a male reaches full physical and sexual maturity, he becomes more and more protective of what he considers "his" territory. His definition of "his" area tends to change, and the boundaries enlarge, until sometimes an entire square block or country mile falls within his territory.
Often, guardians are not aware of this until a tragedy occurs, and their male or another male is severely hurt or even killed. "But he's always so gentle" is a common cry of an upset guardian in these circumstances. And he is - until another male invades property that he considers his own. Then his male territorial instinct overrides any social behavior he may have learned, and he defends his turf, sometimes to the death.

Roaming
Along with this instinct comes roaming behavior. A sexually active male must patrol the boundaries of his property and constantly widen them. In addition, he's always on the lookout for receptive females and, if there is a female in heat within many miles, he'll find her. Along with this comes the potential to be hit by a car or otherwise injured, or become lost. Often, a male hangs around the area for days on end, apparently forgetting that he even has a home. Terrible fights can occur when several males pursue a female in heat, even if she is confined indoors, and the resulting veterinarian bills may be staggering. Research shows us that of all the positive behavior changes that are a result of neutering, roaming shows the greatest degree of change.

Urinating
An un-castrated male may indulge in territorial urine marking - urinating on every upright surface he can find. This is usually related either to a female coming into heat somewhere within his range or another male moving into the neighborhood. You may not be aware of either occurrence, but you will soon know it when your housetrained pet has suddenly "broken training" and is marking up your house. In the absence of other male animals, males may also take out their aggressive territorial protection on humans. Over-protectiveness of family members may manifest itself by growling or nipping at visitors in your home.

Mounting, Climbing, Jumping
Other sexually related behaviors of male dogs can include mounting human legs, climbing up on people, and even knocking children down and climbing on top of them. This is especially frightening and dangerous if a dog is large.

Spraying 
For male cats, a neutered male is less likely to spray (almost all un-neutered male cats spray). They also yowl as if in terrible pain. You may think your cat is in pain and take it to the vet only to find out it is in search of a mate.