Unless you plan on breeding your pet, there are many good reasons to spay/neuter.
There is overwhelming evidence of significantly lower risk of certain types of cancer, such as breast/uterine/ovarian cancer in females, as well as a variety of testosterone driven diseases and cancer in males.
Spaying/neutering your pet also reduces aggression, marking behavior, and aids in training success. The other benefit is your contribution to reducing the unwanted pet population.
