Neutered cats and dogs focus their attention on their human families. On the other hand, unsterilized, unsupervised males roam in search of a mate, risking injury in traffic and in fights with other males. They mark territory by spraying strong-smelling urine on surfaces. Indoors, male dogs may embarrass you by mounting furniture and human legs when stimulated.
Don’t confuse aggressiveness with protectiveness; a neutered dog protects his home and family just as well as an unneutered dog, and many aggression problems can be avoided by early neutering.