Spay/Neuter
There are more animals to adopt than there are people to adopt them.
Spaying and neutering means fewer unwanted animals
Fewer unwanted animals means fewer animals living on the streets, at risk of disease, injury, and starvation
Less disease, injury, and starvation means less pain and suffering
Fewer unwanted animals also means fewer animals living in animal shelters
Fewer animals living in shelters means that fewer innocent lives end in euthanasia