For me it's because I work in rescue and also with shelters. I see how many are killed per day, per week, per month, per year. I am only aware of this death rate within a small region. I was also on the phone with another rescue in Ontario last night that was quoting the euthansia rate there. If you add it up millions of animals are killed all because they were born and sold to people that did not make a life committment to that animal. Then there are the 'accidents' that happen and then again numbers added to the death rate. I cannot even imagine what happens in rural areas.
I guess after 15 years of trying desperately to save them I am all for spay/neuter. I also believe that everyone should be obliged to do so if they are not reputable breeder.
There are those that disagree with me and that is fine. I am in the trenches trying to save them and I also SEE the ones I cannot. This will haunt me forever.
There are medical reasons as well, but my reasons are based on the living, dying and dead.