When my cats were younger and not spayed, they would mark their territory around the house.  This lead to us hitting the cats sometimes or isolating them in a room.  After my cats had gotten spayed they stopped marking their territory and became a lot more playful.  Some people do not tolerate this behavior, and end up abusing and neglecting their pet until they just let it go free into the wild or bring it to a random shelter.  Therefore, you should get your pet spayed or neutered to prevent animal abuse and neglect. Help decrease the number of the pet population because over-population is killing pets.