Why do people get so upset about people who don't spay/ neuter their pets?

I think it should be up to the individual pet owner. I have dogs. I have also bred one dog and then had her spayed after one breeding. My other female, one of the pups I kept from my other dog's litter, is now over five years old. I had planned on breeding her at one time, but I was worried about her size, she is the runt. I planned on getting her spayed, but it was one of those things that just kept getting put off. Now I have a litter of mixed breed pups. She had to have an emergency c-section, no surprise there, but I had her spayed at the same time. I guess people get upset because of the over population of pets, so many animals end up in shelters and then they have a problem finding homes for them. Some shelters are no kill, but a lot of them still put animals down. It is healthier for a animal to be nuetered, they live longer, healthier lives. Male dogs left unnuetered can often develop prostate cancer, and females can develop pyrometria which is an infection of the uterus that they can die from.