Why should owners spay and neuter their pets?
The only reason to not spay or neuter is you want to breed your dog. If you do not breed your dog, my advice is to spay / neuter your pet ASAP. Young pets are ideally spay/ neutered between 6-9 months of age.
Neutered pets are typically much better behaved, and there is a reduced incidence of breast cancer when female pets are spayed, and reduced prostate disease and cancer & testicular cancer in neutered males. The only down side to spay / neuter is a reduction in metabolic rate which makes them more prone to obesity, but this can be easily managed by reducing food intake.