For me, it's two things. 1. I believe my pets are healthier because they are spayed and neutered and the future health benefits and keeping them healthy, and 2. I see so many homeless animals who don't have homes because someone didn't spay or neuter a pet and they couldn't/wouldn't take responsibility to keep the litters that were not wanted.
And as for me adding to the worlds population, at this point in my life I just don't have the patience or the willingness to have a child.
There are only two rules at my house: House rule #1. Cats rule. House rule #2. See rule #1.