Of course, there is the obvious reason of controlling pet overpopulation by preventing accidental pregnancies. However, there are health reasons as well. Unspayed females will have increased chances of pyometra and other reproductive infections and cancer. Unneutered males will have prostatic enlargement, discomfort, and possible infection or cancer of the prostate as well as the testicles. Neutered males are also less likely to roam or mark their territory. Unless you are planning to breed your pet, there is no disadvantage. Having your pet fixed does not make them fat or lazy!