A USA Today (May 7, 2013) article cites that pets who live in the states with the highest rates of spaying/neutering also live the longest. According to the report, neutered male dogs live 18% longer than un-neutered male dogs and spayed female dogs live 23% longer than unspayed female dogs. The report goes on to add that in Mississippi, the lowest-ranking state for pet longevity, 44% of the dogs are not neutered or spayed.
Part of the reduced lifespan of unaltered pets can be attributed to their increased urge to roam, exposing them to fights with other animals, getting struck by cars, and other mishaps.
Another contributor to the increased longevity of altered pets involves the reduced risk of certain types of cancers. Unspayed female cats and dogs have a far greater chance of developing pyometra (a fatal uterine infection), uterine cancer, and other cancers of the reproductive system.
Medical evidence indicates that females spayed before their first heat are typically healthier. (Many veterinarians now sterilize dogs and cats as young as eight weeks of age.)
Male pets who are neutered eliminate their chances of getting testicular cancer, and it is thought they they have lowered rates of prostate cancer, as well.
Getting your pets spayed/neutered will not change their fundamental personality, like their protective instinct.