There are sooooo many reasons to spay and neuter your dog/s. Your dog's health, the lives of homeless dogs, your wallet, your dog's training, the list goes on.
Your dog's health
Spaying helps prevent uterine infections and breast cancer. Spayed females live 23% longer than un-spayed females.
Neutering helps prevent testicular cancer if done before 6 months. neutered males live 18% longer than un-neutered males.
Neutered males will not feel the need to roam to find a mate. Intact males will do anything they can to escape their yard when they feel the need to roam. Not only is it inconvenient to chase Fido around the neighborhood, but it is dangerous. Your dog risks getting hit by cars, getting lost, injured in a fight with other males and many other disasters.
Spaying and neutering does Not make your dog fat.
Intact dogs often spend more time marking territory and looking for a mate then paying attention to their owners. Early neutering and spaying will prevent many, many aggression and overall behavior issues.
The cost of spaying and neutering is WAY cheaper than having to care for a litter or take your dog to the vet after is has gotten in a fight with another dog or has some sort of preventable cancer or infection. Raising a litter of puppies is not cheap, it is not something you can do responsibly and still make a profit.
This is a big one. 6-8 million homeless animals enter shelters every year, not even half are adopted, and the rest are euthanized. more than 2.7 MILLION healthy, adoptable dogs and cats are KILLED every year because there is no room for them. More than 90% of these are strays that come from unplanned litters. This can be prevented so easily and so cheap! Spaying and neutering saves lives. Help the over population problem, don't add to it.